Do not make any __str__ or __repr__ output latex

[eric-wieser]

This follows on from #371, and also hitting Sdop, Pdop, Lt, and Mlt.

This then moves all the duplicated code to printer.py. As a result of this change, only one line of code outside of printer.py uses GaLatexPrinter.latex_flg, and it only does so as a workaround:

https://github.com/pygae/galgebra/blob/0366454409fb0ba5c6c9f3509ec886685ad52fdc/galgebra/mv.py#L91-L94

Note that if __str__ is not defined, it defaults to __repr__ - so we can save some lines by only defining the latter.

[codecov[bot]]

Codecov Report

Merging #376 into master will decrease coverage by 0.04%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #376      +/-   ##
==========================================
- Coverage   77.29%   77.25%   -0.05%     
==========================================
  Files          16       16              
  Lines        4348     4313      -35     
==========================================
- Hits         3361     3332      -29     
+ Misses        987      981       -6     

Impacted Files	Coverage Δ
galgebra/dop.py	89.74% <100.00%> (+0.39%)	:arrow_up:
galgebra/lt.py	57.61% <100.00%> (+0.11%)	:arrow_up:
galgebra/mv.py	75.90% <100.00%> (-0.31%)	:arrow_down:
galgebra/printer.py	77.27% <100.00%> (-0.08%)	:arrow_down:

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