Read and Write GDML colour from/to BDSIM and generally.

[lnevay]

Relatively simple addition to read aux tags in GDML for visualisation options.

Change of LogicalVolume to have no VisOptions instance by default (like PV). Defaults are enforced in the visualiser(s). This is an easier way to tell if one has been explicitly given, or default to materials, or default to supplied grey default. Works as previously expected, but loaded colours take precedence.

This will require the pull request from the test data to be accepted first. See: g4edge/testdata#32

Only with that, can I update the reference SHA1 commit ID here to use the new test file.

GDML with colour can be exported now from BDSIM with once that pull request is also merged.

[codecov]

Codecov Report

Attention: Patch coverage is 90.09901% with 10 lines in your changes missing coverage. Please review.

Project coverage is 74.61%. Comparing base (f9c8bf2) to head (36eacf2).
Report is 20 commits behind head on main.

Files with missing lines	Patch %	Lines
src/pyg4ometry/visualisation/ViewerBase.py	73.07%	7 Missing ⚠️
...rc/pyg4ometry/visualisation/ViewerHierarchyBase.py	75.00%	2 Missing ⚠️
src/pyg4ometry/gdml/Writer.py	92.85%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #228      +/-   ##
==========================================
+ Coverage   74.57%   74.61%   +0.04%     
==========================================
  Files         157      157              
  Lines       22822    22867      +45     
==========================================
+ Hits        17020    17063      +43     
- Misses       5802     5804       +2     

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