Consistently call things "state_vector"

[peterse]

Nitpick: Its hard to keep track of how a method acting on [an object representing a discrete quantum state] is named. This may cause confusion and also stymie autocomplete.

Right now, functions in wave_function.py are named like "state vector" or just "state", while Circuit.final_wavefunction, PauliStrings, PauliSums, and a few linalg transformations are named like "wavefunction". (I acknowledge that this inconsistency has gotten a lot worse following some of my own recent PRs :/ )

[dabacon]

We should probably avoid state as it is too generic and we do have notions of simulator state.

Github needs a "vote on this issue" feature.

At the top level from __init__.py here is the status:

Currently methods in qis use state_vector in method names except validate_normalized_state. state_vector is also used in measure_state_vector and sample_state_vector in sim. 6 total uses.

wavefunction is used in sim for final_wavefunction. In linalg we have subwavefunction (separates state if it is seperable) and wavefunction_partial_trace_as_mixture. 3 total uses.

state is used in plot_state_histogram and in simulator for simulator state.

Let's vote on this at the next Cirq Cync.

[dabacon]

At Cirq Cync we decided that we should use "state_vector".

From notes:
"all API names should say state_vector, docstrings should say state_vector first"

To do this we will need to first deprecate wavefunction methods and add state_vector methods. This will go out in 0.9.0. For 0.10.0 we will then remove deprecated methods.

[dabacon]

Fixed in #3022 but will take deprecation process to fully clean up.