Fix up how `rep` is used and set

[gordonwatts]

• Streamline where .rep is checked for staleness (only one place needed)! (Fixes #163)
• Remove usage of _result (Fixes #167)
• Use methods to do set and get of the ast node's representation (Fixes #41)
• Fix up a bunch of flake8 errors
• Split up how we run CI so we do flake8 first, and then everything else after.

[codecov[bot]]

Codecov Report

Merging #170 (0194e0e) into master (df8a71c) will increase coverage by 0.14%. The diff coverage is 92.85%.

@@            Coverage Diff             @@
##           master     #170      +/-   ##
==========================================
+ Coverage   92.23%   92.38%   +0.14%     
==========================================
  Files          24       24              
  Lines        1314     1287      -27     
  Branches      146      142       -4     
==========================================
- Hits         1212     1189      -23     
+ Misses         70       67       -3     
+ Partials       32       31       -1     

Flag	Coverage Δ
unittests	92.38% <92.85%> (+0.14%)	:arrow_up:

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
func_adl_xAOD/common/ast_to_cpp_translator.py	85.77% <91.83%> (-0.09%)	:arrow_down:
func_adl_xAOD/common/cpp_representation.py	95.37% <100.00%> (+0.17%)	:arrow_up:
func_adl_xAOD/common/executor.py	98.83% <100.00%> (ø)

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