Serialization of TwoQubitQPDGate should not decompose it into two SingleQubitQPDGates

[garrison]

Assuming #417 is fixed upon merging #442, it will be possible to serialize the QPDGates. However, in the course of serializing a TwoQubitQPDGate, it will first be decomposed to two SingleQubitQPDGates. This is probably not desirable: it would almost certainly be better for a TwoQubitQPDGate to make it through a serialization/deserialization process unchanged.

To fix this, we can make TwoQubitQPDGate._define do nothing if basis_id is None, just like in SingleQubitQPDGate._define. However, we will need to provide users a straightforward way to decompose all (or perhaps some?) TwoQubitQPDGates in a circuit into SingleQubitQPDGates. Right now this can be accomplished with circuit.decompose(TwoQubitQPDGate), but this procedure would no longer work with the change suggested here. Whatever we do, we should be sure to document it carefully in the release notes.