Layer formulation

[carldlaird]

This PR merges work from many different people.

The key item added was @fracek reworking of the network definition to include individual objects for layers and to allow for multi-dimensional inputs and outputs. It also includes reworking of other parts of the code to update them to this new foundation.

This also added gbt and some preliminary implementations of different notebooks.

[codecov]

Codecov Report

Merging #24 (56845c8) into main (2c38689) will decrease coverage by 2.30%.
The diff coverage is 89.27%.

@@            Coverage Diff             @@
##             main      #24      +/-   ##
==========================================
- Coverage   91.10%   88.79%   -2.31%     
==========================================
  Files          15       23       +8     
  Lines         854     1071     +217     
  Branches      159      160       +1     
==========================================
+ Hits          778      951     +173     
- Misses         51       98      +47     
+ Partials       25       22       -3     

Impacted Files	Coverage Δ
src/omlt/io/onnx.py	47.61% <38.88%> (-44.78%)	⬇️
src/omlt/neuralnet/layers/full_space.py	60.60% <60.60%> (ø)
src/omlt/neuralnet/layer.py	73.33% <73.33%> (ø)
src/omlt/neuralnet/network_definition.py	89.65% <88.46%> (-4.99%)	⬇️
src/omlt/io/onnx_parser.py	93.16% <93.16%> (ø)
src/omlt/neuralnet/nn_formulation.py	95.31% <95.31%> (ø)
src/omlt/neuralnet/activations/relu.py	96.77% <96.77%> (ø)
src/omlt/block.py	100.00% <100.00%> (+7.27%)	⬆️
src/omlt/formulation.py	92.06% <100.00%> (+3.82%)	⬆️
src/omlt/gbt/__init__.py	100.00% <100.00%> (ø)
... and 13 more

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[jalving]

See comments. PR looks good to me.

We should update the readme example before merging or at least very shortly after merging.
I updated the readme example. It works locally, but we should look into setting up a test for that as well.