Add quimb_gate, which returns a single quimb gate

[garrison]

This introduces a quimb_gate function that is meant to return a single gate. Right now it handles xxplusyygate as a special case in the old _gen_quimb_gates function because it maps to multiple quimb gates, but we can factor this out upon the next quimb release, as xxplusyygate will then be available in quimb (as a givens2 rotation, IIRC).

I think the main design decision here it to decide whether Barrier should map to None or an identity ("IDEN").

I tried adding some tests, but it was difficult to add meaningful tests when even the following fails

def test_quimb_gate():
    def construct():
        return quimb.tensor.Gate("IDEN", params=[], qubits=(0,))
    assert construct() == construct()

Fixes #3.