FIL to import categorical models from treelite (rapidsai#4173)

* importing forests with categorical nodes from Treelite
* python tests for lightgbm forests fit on categorical features and containing categorical nodes as well as numerical nodes in the same forest
* above tests include multiclass (GBDT) tests
* sklearn internally transforms categorical features into numerical: [RandomForestClassifier](https://scikit-learn.org/0.24/modules/generated/sklearn.ensemble.RandomForestClassifier.html#sklearn.ensemble.RandomForestClassifier.fit) [GradientBoostingClassifier](https://scikit-learn.org/0.24/modules/generated/sklearn.ensemble.GradientBoostingClassifier.html#sklearn.ensemble.GradientBoostingClassifier.fit)
* (cuml RF tests can come separately, if needed)
* (xgboost tests can come separately, if needed)

Authors:
  - Levs Dolgovs (https://github.com/levsnv)
  - Dante Gama Dessavre (https://github.com/dantegd)

Approvers:
  - https://github.com/Salonijain27
  - Andy Adinets (https://github.com/canonizer)
  - William Hicks (https://github.com/wphicks)
  - Dante Gama Dessavre (https://github.com/dantegd)

URL: rapidsai#4173