Add support for different TensorNetwork backends

[grmlarose]

For example, tensorflow is not supported because, e.g., mpsim.gates.is_unitary assumes the node object has a .conj() method in the final line

return np.allclose(
    gate.conj().T @ gate, np.identity(gate.shape[0]), atol=1e-5
)

which is unique to the numpy backend. (This also explicitly uses numpy in other locations.)

To add support for different backends:

1. Understand the best way to do this with tensornetwork.
2. Add several tests for different backends.