2.1: Remove c++ driver classes for high-order methods

Created by: gcarleo

As done in #276 , we should remove all the C++ high-level applications (variational, supervised, unsupervised), and change them to new python drivers. The new python drivers should accept as much as possible the same arguments of the old C++ drivers, to ease the transition. Full compatibility with the old drivers should be guarantueed, for example using different namespaces for the new drivers. The new drivers should be just in the netket main module (say netket.Vmc, netket.Supervised etc)