Implemented Bose-Hubbard hamiltonian

Created by: Mikelesc

Hello, we are trying to check the Bose-Hubbard model for some trivial cases using the NetKet implementation of this model and the example code provided. For example for 0 hopping and just 1 boson we expect the gs energy to be zero (we were assuming the hamiltonian as in the Hiroki Saito's paper: https://arxiv.org/abs/1707.09723 ). We are not getting the expected values so we would like to know the exact expression for the hamiltonian that it's implemented in NetKet.