Type annotate qcodes.math.*, make FieldVector's attributes always computed

[astafan8]

todo until completion:

• [ ] rework the logic regarding _compute_unknowns method so that a FieldVector instance ALWAYS has ALL of its coordinates calculated (currently it is possible to end up with a FieldVector that has some of its attributes equal to None)
• [ ] remove T TypeVar completely becuase mypy complained when it was used to annotate dunder methods :( or use T such that mypy understands the typing of dunders