More features for CNN in OMLT

[zshiqiang]

Current implementation of convolutional 2D layer assumes (1) no paddings (2) no dilations (3) zero biases.

To add these features, we need:

• parameters corresponding to those features in the Layer2D definition: https://github.com/cog-imperial/OMLT/blob/main/src/omlt/neuralnet/layer.py

• a bias term in the Layer2D constraints: https://github.com/cog-imperial/OMLT/blob/main/src/omlt/neuralnet/layers/full_space.py

By generalizing function kernel_index_with_input_indexes in line 258 of https://github.com/cog-imperial/OMLT/blob/main/src/omlt/neuralnet/layer.py, it's possible to support more pooling and convolutional layers (e.g., 1D, 3D).