Explain and possibly refine the concepts of "generator" and "arguments"

[pablolh]

Those properties tag gate semantics with how they are obtained.

For instance a BlochSphereRotation(q, axis=(1, 0, 0), angle=pi) can be

1. anonymous, as obtained from merging two other gates (in which case there is no generator nor argument)
2. obtained from x q, modulo irrelevant global phase difference (generator = x, arguments = (q,))
3. obtained from rx q, pi (generator = rx, arguments = (q, pi)