Add magic rounding support for (1,1,p) and (2,1,p) QRACs

[areeq-hasan]

Summary

Details and comments

[garrison]

This is great, thanks!

My only additional comment is that I think vars_per_qubit should be changed to _vars_per_qubit everywhere within RoundingContext (it can remain as it is, however, in magic_rounding.py), both as an argument name in the constructor and in its name as assigned on the object. It really only exists to make our lives easier in test_magic_rounding.py and will be dropped as soon as we refactor to have adaptive QRACs. Since we don't plan to support this new API indefinitely, let's introduce it as protected instead of public (i.e., with the leading underscore). The only files that should need modified as part of this tweak are rounding_common.py and test_magic_rounding.py.

[garrison]

I added tests in 2a9e176.

There's no explicit test of weighted sampling for (2,1,p) and (1,1,1) QRACs -- at least, not yet. I think that's the only thing missing (and I don't think I'd hold up merging to wait on it).