refactor quantile regressor classes

[lbventura]

This is a small refactoring to avoid duplicate code at the level of CBMultiplicativeQuantileRegressor and CBAdditiveQuantileRegressor. This design (see below) could be improved though by includingCBMultiplicativeGenericRegressor and CBAdditiveGenericRegressor . This is, unfortunately, not possible ATM because the class structure relies on inheritance to implement methods. Using composition instead would likely clean up the design, I will attempt to do this in a follow-up PR.

[FelixWick]

But you could simply add another middle base class CBGenericRegression (same hierarchy level as CBQuantileRegression) from which CBMultiplicativeGenericRegression and CBAdditiveGenericRegression would inherit (in the same way CBMultiplicativeQuantileRegression and CBAdditiveQuantileRegression are inheriting from CBQuantileRegression), couldn't you?