Create X-YZ decomposer for quantify-scheduler

[pablolh]

Quantify-scheduler's gate library supports RZ and RXY. This last gate can represent any bloch sphere rotation of any angle provided the axis is in the plan (X, Y).

Currently we use the ZYZ decomposition to decompose gates for quantify-scheduler. This requires 3 gates.

But since QS supports RXY, it is possible that in a lot (all?) cases, only 2 of those gates are required to describe an arbitrary single-qubit unitary.

Goal of this issue:

• investigate if there exists a mathematical unitary decomposition for 2x2 unitaries into either RZ.RXY or RXY.RZ
• implement corresponding decomposer and use it for quantify-scheduler

[juanboschero]

There is ZX decomposition which may be interesting https://arxiv.org/pdf/1602.04744.pdf

[juanboschero]

This requires a defined RXY gate in the gate definitions

[rturrado]

OK, but we can leave that out at the moment as an implementation detail.

I would focus on the first bullet: investigate if there exists a mathematical unitary decomposition for 2x2 unitaries into either RZ.RXY or RXY.RZ.