Allow for viz_stops to be used without stop_code column for MGTFS

[CBROWN-ONS]

…at: add new tests

Description

This PR fixes an issue where gtfs::multi_validation::MultiGtfsInstance.viz_stops() can not be used if any of the GTFS are missing a stop_code column. This has been fixed by synthesizing a stop_code column if it is not present.

Fixes
Fixes #234

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

How Has This Been Tested?

Test configuration details:

• OS: Windows 10
• Python version: 3.9.13
• Java version: N/A
• Python management system: Conda

Checklist:

• My code follows the intended structure of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published in downstream modules

Additional comments

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (aa98494) 97.62% compared to head (fc6688e) 97.63%.

Additional details and impacted files

@@           Coverage Diff           @@
##              dev     #236   +/-   ##
=======================================
  Coverage   97.62%   97.63%           
=======================================
  Files          20       20           
  Lines        1770     1775    +5     
=======================================
+ Hits         1728     1733    +5     
  Misses         42       42           

Flag	Coverage Δ
unittests	97.63% <100.00%> (+<0.01%)	⬆️

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[r-leyshon]

Good approach - placeholder stop_code column is created but the original feed remains untouched. Test to prove the enhancement behaviour.