Add direct interfaces to LGBM, CatBoost, etc. to allow for categorical vars

[ThebTron]

Currently ONNX doesn't support categorical vars which limits the use of tree ensembles in OMLT. By directly interfacing with these libraries, we can add support for categorical vars.

[ThebTron]

Some of the functions needed to create the optimization formulation of the tree ensemble with categorical vars are given in the following public branch: https://github.com/cog-imperial/OMLT/tree/cat_var_support