Get rid of netket::Ising and other hard-coded hamiltonians in c++

Created by: gcarleo

In the spirit of #199 (closed) , we can safely remove the Ising and Heisenberg hamiltonians from the C++ classes, since those are nothing but specific cases of more general hamiltonians (basically they are just LocalOperators or even GraphOperator). Convenient constructors can be defined in few lines of python, and moved to the python part of the library.

The BoseHubbard hamiltonian is a bit trickier, we can keep it as it is for the moment.