Enhanced CNN Support

[pulsipher]

Currently, 2D convolutional layers can be read into OMLT. To better support CNNs, the following layers should also be accommodated:

• Pooling layers (i.e., the ones summarized here)
• 1D convolutional layers (maybe 3D?)

These additions would enable the use of typical 1D and 2D CNNs (maybe 3D if desired...).

[zshiqiang]

Many thanks for these ideas! We raise an issue about enhancing CNN in OMLT, please see #129.