Add Full Trees Support to KhiopsRegressor

[popescu-v]

Currently, setting n_trees to a value strictly greater than 0 on a KhiopsRegressor instance leads to a warning which states that trees are not supported for regression. This statement is no longer true since Khiops 11.0.0. However, n_trees is well propagated to the Core API train_predictor as max_trees.

Hence, the goal of this issue is to:

• remove the above said warning
• add full testing that n_trees is passed as max_trees to the Core API in the regression case.