[ENH] Add the hook on forest estimator

[xuyxu]

This PR enables one to get the forest in the model more efficiently. It turns out that many people are interested to investigate each forest estimator.

To get the forest you want, simply use the following method:

model.get_forest(layer_idx, est_idx, forest_type)

which returns the est_idx-th forest in the layer_idx-th cascade layer of the type: forest_type.

[xuyxu]

LGTM