init - made InteractionHamiltonian into an interface, now with two
implementations available (FastIH and SlowIH). The FastIH is verified to
produce the same results, but MUCH faster!
modified - added some Combinatoric functions to make the calculation of
the FockState label based on entries fast.
modified - changed the InteractionHamiltonian used in the QuantumState
to the FastIH, as it was verified to work.