Include boson and couple them to fermions

This package currently finds matrix and operator representation of fermions only. It would be useful to include bosons, and their coupling to fermions, to simulate hybrid superconducting qubits.

There are several python packages that try to do that, but I don't think any of them uses sympy functionalities. Also, they typically rely on truncating the boson Hilbert space, which I am not sure if we can get around.