renamed the HubbardModelVMC class to ModelManager
renamed the SingleParticleHamiltonian class to VariationalHamiltonian
the WMatrix now no longer has a pointer to the lattice (it was kind of superfluous)
merged latest changes from master
renamed the irreducible index relations to irreducible _spindex_ relations
changed the filename of the 2d square lattice class to lattice_2dsqr.*
merged name changes from master