Expression fix

[stewartboogert]

Class for GDML expressions was complex. Laurie refactored on bitbucket. Same code here just with pre-commit checks.

[codecov]

Codecov Report

Merging #40 (515fef5) into main (b8779e7) will decrease coverage by 0.04%.
The diff coverage is 39.19%.

@@            Coverage Diff             @@
##             main      #40      +/-   ##
==========================================
- Coverage   24.27%   24.23%   -0.04%     
==========================================
  Files         145      144       -1     
  Lines       21045    21019      -26     
==========================================
- Hits         5108     5095      -13     
+ Misses      15937    15924      -13     

Impacted Files	Coverage Δ
...g4ometry/gdml/GdmlExpression/GdmlExpressionEval.py	63.84% <ø> (ø)
...4ometry/gdml/GdmlExpression/GdmlExpressionLexer.py	100.00% <ø> (ø)
...ometry/gdml/GdmlExpression/GdmlExpressionParser.py	62.12% <ø> (ø)
...metry/gdml/GdmlExpression/GdmlExpressionVisitor.py	53.12% <ø> (ø)
src/pyg4ometry/gdml/GdmlExpression/__init__.py	100.00% <ø> (ø)
src/pyg4ometry/gdml/Reader.py	3.88% <0.00%> (-0.01%)	⬇️
src/pyg4ometry/geant4/DivisionVolume.py	6.81% <0.00%> (ø)
src/pyg4ometry/geant4/__init__.py	100.00% <ø> (ø)
src/pyg4ometry/geant4/solid/Scaled.py	27.58% <ø> (ø)
src/pyg4ometry/geant4/Registry.py	24.81% <21.42%> (+0.04%)	⬆️
... and 3 more

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