Replacing assert statements with Exceptions

[jezsadler]

Replaced assert statements across several files with ValueError or TypeError messages, which should be more informative.

Included unit tests for many of these - some of the error states were difficult to induce.

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[codecov]

Codecov Report

Attention: 78 lines in your changes are missing coverage. Please review.

Comparison is base (b9600e2) 95.95% compared to head (4a83a22) 91.89%.

❗ Current head 4a83a22 differs from pull request most recent head 7d660c4. Consider uploading reports for the commit 7d660c4 to get more accurate results

Files	Patch %	Lines
src/omlt/io/onnx_parser.py	45.94%	20 Missing and 20 partials ⚠️
src/omlt/neuralnet/layer.py	60.97%	10 Missing and 6 partials ⚠️
src/omlt/gbt/model.py	20.00%	5 Missing and 3 partials ⚠️
src/omlt/neuralnet/nn_formulation.py	66.66%	4 Missing and 4 partials ⚠️
src/omlt/gbt/gbt_formulation.py	0.00%	3 Missing and 3 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #137      +/-   ##
==========================================
- Coverage   95.95%   91.89%   -4.06%     
==========================================
  Files          29       29              
  Lines        1656     1752      +96     
  Branches      255      330      +75     
==========================================
+ Hits         1589     1610      +21     
- Misses         35       76      +41     
- Partials       32       66      +34     

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[rmisener]

Thanks so much @jezsadler for these super useful changes. I'm going to give the team one last chance to object during the call today and then merge this.