[WIP] Test if SimplePDF solve the pdf build issue

[cuihantao]

The build statuses are found here: https://readthedocs.org/projects/andes/builds/

[jinningwang]

I don't have meaningful insight into this issue, I might need to do trial and error with RTD by this PR...

[jinningwang]

I failed to have a meaningful fix on this issue. I've tried:

1. Running make latexpdf locally, run into error. The error message is attached at last.
2. Using RTD docker to debug PDF generation locally, but failed with running part because ANDES source files are not copied correctly.

I might try method 2 later on.

make latexpdf error message:

=== TeX engine is 'XeTeX'
Latexmk: Index file 'andes.idx' was written
Latexmk: References changed.
Latexmk: References changed.
Latexmk: Log file says output to 'andes.pdf'
Latexmk: List of undefined refs and citations:
  Citation `getting_started/formats/disturbance:milano2010' on page 44 undefined on input line 4197
  Citation `getting_started/testcases/index:ei' on page 45 undefined on input line 4284
  Citation `getting_started/testcases/index:gb' on page 45 undefined on input line 4279
  Citation `getting_started/testcases/index:ieee' on page 45 undefined on input line 4288
  Citation `getting_started/testcases/index:matpower' on page 45 undefined on input line 4297
  Citation `getting_started/testcases/index:nordic' on page 45 undefined on input line 4301
  Citation `getting_started/testcases/index:pjm5' on page 45 undefined on input line 4275
  Citation `getting_started/testcases/index:psat' on page 45 undefined on input line 4317
  Citation `getting_started/testcases/index:pst' on page 45 undefined on input line 4307
  Citation `getting_started/testcases/index:rlgc' on page 45 undefined on input line 4292
  Citation `getting_started/testcases/index:sauer' on page 45 undefined on input line 4271
  Citation `getting_started/testcases/index:wecc' on page 45 undefined on input line 4312
  Citation `index:cui2021' on page 1 undefined on input line 288
  Reference `_generated/andes.cli:module-andes.cli' on page 875 undefined on input line 169818
  Reference `_generated/andes.interop.gridcal:module-andes.interop.gridcal' on page 875 undefined on input line 169820
  Reference `_generated/andes.interop.matpower:module-andes.interop.matpower' on page 875 undefined on input line 169821
  Reference `_generated/andes.interop.pandapower:module-andes.interop.pandapower' on page 875 undefined on input line 169822
  Reference `_generated/andes.interop.pypowsybl:module-andes.interop.pypowsybl' on page 875 undefined on input line 169823
  Reference `_generated/andes.interop:module-andes.interop' on page 875 undefined on input line 169819
  Reference `_generated/andes.io.json:module-andes.io.json' on page 875 undefined on input line 169825
  Reference `_generated/andes.io.matpower:module-andes.io.matpower' on page 875 undefined on input line 169826
  Reference `_generated/andes.io.psse:module-andes.io.psse' on page 875 undefined on input line 169827
  Reference `_generated/andes.io.streaming:module-andes.io.streaming' on page 875 undefined on input line 169828
  Reference `_generated/andes.io.txt:module-andes.io.txt' on page 875 undefined on input line 169829
  Reference `_generated/andes.io.xlsx:module-andes.io.xlsx' on page 875 undefined on input line 169830
  Reference `_generated/andes.io:module-andes.io' on page 875 undefined on input line 169824
  Reference `_generated/andes.main:module-andes.main' on page 875 undefined on input line 169831
  Reference `_generated/andes.plot:module-andes.plot' on page 875 undefined on input line 169832
  Reference `_generated/andes.routines.base:module-andes.routines.base' on page 875 undefined on input line 169834
  Reference `_generated/andes.routines.criteria:module-andes.routines.criteria' on page 875 undefined on input line 169835
  Reference `_generated/andes.routines.daeint:module-andes.routines.daeint' on page 875 undefined on input line 169836
  Reference `_generated/andes.routines.eig:module-andes.routines.eig' on page 875 undefined on input line 169837
  Reference `_generated/andes.routines.pflow:module-andes.routines.pflow' on page 875 undefined on input line 169838
  Reference `_generated/andes.routines.tds:module-andes.routines.tds' on page 875 undefined on input line 169839
  Reference `_generated/andes.routines:module-andes.routines' on page 875 undefined on input line 169833
  Reference `_generated/andes.system:module-andes.system' on page 875 undefined on input line 169840
  Reference `_generated/andes.utils.paths:module-andes.utils.paths' on page 875 undefined on input line 169841
  Reference `_generated/andes.utils.snapshot:module-andes.utils.snapshot' on page 875 undefined on input line 169842
  Reference `_generated/andes.utils.widgets:module-andes.utils.widgets' on page 875 undefined on input line 169843
  Reference `_generated/andes.variables.dae:module-andes.variables.dae' on page 875 undefined on input line 169845
  Reference `_generated/andes.variables.fileman:module-andes.variables.fileman' on page 875 undefined on input line 169846
  Reference `_generated/andes.variables.report:module-andes.variables.report' on page 875 undefined on input line 169847
  Reference `_generated/andes.variables:module-andes.variables' on page 875 undefined on input line 169844
Latexmk: Summary of warnings from last run of *latex:
  Latex failed to resolve 30 reference(s)
  =====Latex reported missing or unavailable character(s).
=====See log file for details.
  Latex failed to resolve 13 citation(s)
Latexmk: Errors, so I did not complete making targets
Collected error summary (may duplicate other messages):
  pdflatex: Command for 'pdflatex' gave return code 1
      Refer to 'andes.log' for details
Latexmk: Use the -f option to force complete processing,
 unless error was exceeding maximum runs, or warnings treated as errors.
make[1]: *** [andes.pdf] Error 12
make: *** [latexpdf] Error 2