add --nogen option to run tests on pregenerated code

[tlambert03]

This adds a --nogen option to the pytest command that lets you run tests on the pregenerated model code. So you can:

1. run python -m ome_autogen to generate the model
2. muck about manually with the generated model at src/ome_types/model
3. run pytest with pytest --nogen to test against your manually edited code.
   • ... use pytest --nogen --runxfail to see errors on skipped files
   • ... use (e.g.) pytest --nogen --runxfail -k spim to test a single xml

[codecov-commenter]

Codecov Report

Merging #14 into master will increase coverage by 0.02%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master      #14      +/-   ##
==========================================
+ Coverage   95.20%   95.22%   +0.02%     
==========================================
  Files           1        1              
  Lines         396      398       +2     
==========================================
+ Hits          377      379       +2     
  Misses         19       19              

Impacted Files	Coverage Δ
src/ome_autogen.py	95.22% <100.00%> (+0.02%)	⬆️

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