QPDGate serialization meta-issue

[garrison]

There are still some issues with QPDGate serialization via qiskit.qpy once #417 is resolved by #442. I'm opening this issue to collect them and to create a space for centralized discussion. (These issues are closely related to one another, and it is likely that they should be considered together for any potential solution, as @caleb-johnson suggested at #417 (comment).)

• TwoQubitQPDGate with basis_id=None gets serialized into two SingleQubitQPDGates: Serialization of TwoQubitQPDGate should not decompose it into two SingleQubitQPDGates #443
• SingleQubitQPDGate with basis is not None gets realized into concrete gates during serialization (@caleb-johnson's comment)
• deserializing a QPDGate results instead in a concrete instantiation of an object of type Instruction: Deserializing a SingleQubitQPDGate results in a generic Instruction #444

[garrison]

After some further discussion, it's possible that users should really be using pickle instead of qpy if they need things to work as suggested above. Closing for now, and will re-open if necessary.