Add support for sparse input to the Bagging models

Bagging models as implemented in #2375 currently only support dense array-like input. We need add support for CSR or CSC input.

Depending on whether sample-bagging (without weights) and / or feature-bagging are enabled and the kind of input data representation we get some copy of re-samples of the input data are likely to be required (by calling tocsr() or tocsc()).