Implementation of fermionic hilbert and operator

Created by: jwnys

This is an implementation of discrete fermionic degrees of freedom by @imi-hub and myself. It contains the hilbert space and an operator. The operator handles the minus signs coming from the exchange symmetry, while the hilbert space stores occupation numbers only. This follows e.g. the way qiskit and openfermion would implement things. For the operator, we follow the implementation of openfermion and added an 'from_openfermion' function to create operators. This allows us to use all their hamiltonians, such as Fermi-Hubbard. The hilbert space is not optimized for fixed number of fermions (since it inherits from HomogeneousHilbert), for that we might need to add another Hilbert implementation.

FYI: formatting will fail because flake8 and flakehell are giving me hell.