Fix H columns for single-filter runs

[astronomerritt]

Fixes #760.

This makes sure columns are consistent across single and multi-filter runs by ensuring that both have the 'H_[mainfilter]' and 'H_filter' columns in output. In the case of a single filter run, these two columns will be identical.

(We assume that in the case of a single-filter run, the user will have supplied H in the relevant filter.)

The unit test for PPCalculateApparentMagnitude now also checks to make sure both columns are created when a single observing filter is provided.

Review Checklist for Source Code Changes

• Does pip install still work?
• Have you written a unit test for any new functions?
• Do all the units tests run successfully?
• Does Sorcha run successfully on a test set of input files/databases?
• Have you used black on the files you have updated to confirm python programming style guide enforcement?

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (f3ed452) 74.82% compared to head (a27f0cf) 74.82%.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #768   +/-   ##
=======================================
  Coverage   74.82%   74.82%           
=======================================
  Files          66       66           
  Lines        3023     3023           
=======================================
  Hits         2262     2262           
  Misses        761      761           

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