Circuit.q_registers and Circuit.c_registers are broken for circuits whose units are not organized as registers

[cqc-alec]

For example, if a circuit contains a single qubit Qubit("q", 3) then q_registers will return [QubitRegister("q", 1)]; but QubitRegister("q", 1) contains Qubit("q", 0) which is not in the circuit.

I suppose it should return an empty list in this case, or else raise an exception.