core: add quantum operation attribute superclasses

[alexarice]

Prototype for new "gate" superclass. Idea is to capture

• Quantum gates
• Measurements
• Parametrised things

all in one class.

I'm not really sure if there's a way to test it until adding some gates.

[kimworrall]

Seems sensible. Added some nitpicks to the documentation to make it clearer.

[kimworrall]

Before we go ahead I'd like to test some different examples.
I think the best to start with is a parameterised unitary and gate parameterised measurement.

[alexarice]

Before we go ahead I'd like to test some different examples. I think the best to start with is a parameterised unitary and gate parameterised measurement.

I don't understand this comment, but no rush on this

[alexarice]

I'd like to be able to eventually capture:

• Gates
• Angle parametrised gates
• Classically conditioned gates
• Combinators to allow adding classical conditioning or quantum conditioning to a gate
• PVMs
• Stabiliser measurements (i.e. measurement of Pauli strings)

The current system can do that, and I thought this is what we converged to on the board, but happy to head alternatives.

[alexarice]

I'll also reformat once we're done with small changes