Add a charge basis

[amilsted]

This basis is a truncated U(1) irrep basis intended to represent quantized charge. The charge operator is defined as are the conjugate (shift) operators e^(ikφ) associated with the U(1) degree of freedom φ.

Could do a minor version bump after this as we will export a bunch of new functions.

[Krastanov]

This looks good to me, but I would really appreciate an example of an interesting Hamiltonian for a system of actual interest dropped somewhere in a docstring, if you have the bandwidth for this. Otherwise I feel someone would see it in the documentation but struggle to think of a situation where they would want to use it.

[Krastanov]

I restarted one of the tests to see whether codecov would pick things up now that the token is properly set.

[amilsted]

@Krastanov I have added a transmon Hamiltonian as an example (also hooked up the docs).

[Krastanov]

Thanks! I am in favor of merging!

[amilsted]

@david-pl Are you happy with this?

[amilsted]

@Krastanov Any objections to a minor version bump?

[Krastanov]

not at all -- you never need to ask me that, I am always in favor of quick releases after each PR

[amilsted]

Right, I just meant minor vs. patch. If we do minor, we'll need to bump compat on QO.jl too, but I think it's worth doing when the exported functions change a bunch. For example, this will help me migrate private code to use the charge basis.

[Krastanov]

Apologies, I misunderstood. Yes, I believe your plan makes sense.

[amilsted]

Ok, great. I will also bump the QO.jl minor version, since it reexports QOB.