Generalize _Uprep

[JonathonMisiewicz]

AnsatzAlgorithm methods currently initialize the qubit register in the zero state and then apply a _Uprep circuit to prepare the general reference. While this is most relevant for an actual quantum device, there's been a request from one of Scuseria's students to generalize this to allow initializing the state directly without specifying a state that constructs it. I agree with this on principle, but there are two caveats:

1. Not all methods allow for this. In particular, PQE requires an operator pool that satisfies K^2 Phi = - Phi for each K in the pool. Obviously, this depends on reference state Phi.
2. There should be a warning that to simulate an actual quantum device, you really do need to be able to prepare your state of interest via circuit.

[JonathonMisiewicz]

I'd like to see this knocked out tomorrow.