Fixed NRT issues and tidied code.

[las-nsc]

Fixes #332 and #333

While I was at it, I couldn't help myself from modernising and tidying up the code:

• Avoid hard-coded strings.
• new()
• pattern matching
• helper methods to avoid duplication

Removed about 500 lines of code.

All tests passing except XsdElsterDatenabholung5.CanCompileClasses due to missing files.

[codecov]

Codecov Report

Merging #336 (4a9afce) into master (85dcbb4) will decrease coverage by 0.25%.
The diff coverage is 94.61%.

@@            Coverage Diff             @@
##           master     #336      +/-   ##
==========================================
- Coverage   95.78%   95.53%   -0.26%     
==========================================
  Files          19       19              
  Lines        3371     3021     -350     
  Branches      496      464      -32     
==========================================
- Hits         3229     2886     -343     
+ Misses         82       76       -6     
+ Partials       60       59       -1     

Impacted Files	Coverage Δ
XmlSchemaClassGenerator/CodeUtilities.cs	92.16% <90.82%> (-2.54%)	⬇️
XmlSchemaClassGenerator/RestrictionModel.cs	89.36% <95.00%> (-5.29%)	⬇️
XmlSchemaClassGenerator/TypeModel.cs	96.82% <96.02%> (+0.02%)	⬆️
XmlSchemaClassGenerator/ModelBuilder.cs	97.69% <100.00%> (ø)

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[mganss]

Thanks. I don't see any failing tests. Can this be merged?

[las-nsc]

I'm happy to if you're not bothered about the -0.26% code coverage