Separate summarise-herg-qc script command line parsing

[MichaelClerx]

Make it callable in a unit test

[codecov]

Codecov Report

❌ Patch coverage is 59.28144% with 68 lines in your changes missing coverage. Please review.
✅ Project coverage is 68.44%. Comparing base (60e55ff) to head (031e88b).
⚠️ Report is 10 commits behind head on main.

Files with missing lines	Patch %	Lines
pcpostprocess/scripts/summarise_herg_export.py	54.60%	64 Missing ⚠️
tests/test_scripts.py	87.50%	3 Missing ⚠️
pcpostprocess/scripts/__main__.py	0.00%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##             main     #118       +/-   ##
===========================================
+ Coverage   37.79%   68.44%   +30.64%     
===========================================
  Files          14       15        +1     
  Lines        2114     2123        +9     
===========================================
+ Hits          799     1453      +654     
+ Misses       1315      670      -645     

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