Adopt Quantity type annotations that preserve units #1413

For many of our functions, we use annotations for units:

def func(x: u.m, t: u.s) -> u.m / u.s:
    return (x / t).to(u.m / u.s)

These are not type hint annotations, since the expected value is a Quantity in most situations. Instead the annotations are used by decorators like our validators and astropy.units.quantity_input. Unfortunately, this means that we can't use static type checkers like mypy to check PlasmaPy code. 😿 Using mypy has the potential to help us find bugs, as well as errors in existing type hint annotations (#268).

Astropy now allows for Quantity type annotations that preserve units. Both typing.Union and typing.Optional are supported.

def func(x: Quantity[u.kpc]) -> Quantity[u.m]:
    return x << u.m

We should migrate towards using Quantity annotations that preserve units in PlasmaPy. However, before we do this, we would need to change our validators to make them work with this style of annotation.