Partition-based formulation for CNN

[rmisener]

The partition-based formulation currently only works for dense neural networks. We would like to extend the partition-based formulation to additionally apply to CNNs.

A research paper describing the partition-based formulation is here: https://proceedings.neurips.cc/paper/2021/hash/17f98ddf040204eda0af36a108cbdea4-Abstract.html

The challenge is to write code equivalent to partition_based.py but with the same careful indexing that had to happen here:

OMLT/src/omlt/neuralnet/layer.py

Line 171 in 683caa7

class ConvLayer(Layer):

Even better: generalize the partition-based formulation for both fully-dense NNs and CNNs and thereby avoid having the same tricky indexing in two places.