Make J and use of sign rule configurable for Heisenberg Hamiltonian
Created by: femtobit
Ising
has configurable J
, so why not also make it configurable for Heisenberg
(even though in the static case it's only is a rescaling of the energy).
Also, while for VMC on a bipartite lattice the sign rule is crucial, adding the option of disabling it can be useful (e.g., to see its influence).