Make validators compatible with Quantity type annotations that preserve units

[namurphy]

In #1413, I raised an issue to adopt astropy.units.Quantity Quantity type annotations that preserve units.

Before we can resolve #1413, we would need to make the decorators in plasmapy.utils.decorators.validators compatible with annotations like:

import astropy.units as u
from typing import Optional, Union

def f(x: u.Optional[u.Quantity[u.m]]) -> u.Quantity[u.m]):
    ...

def g(x: Union[u.Quantity[u.K], u.Quantity[u.eV]]) -> Optional[u.Quantity[u.K]]:
    ...

This might be a good first issue for someone who is familiar with Python but not PlasmaPy.

[RAJAGOPALAN-GANGADHARAN]

@namurphy Can I give this a shot?

[rocco8773]

@namurphy and I have gone back and forth about this implementation. I honestly don't like Quantity's type annotation in its current form because it will make the signature line in our code and in our docs really messy. With that said, I'm still up for supporting it, but only if we still support the older style of typing. This issue would be additional functionality and not replacement functionality.

To implement this, you are actually going to have to modify CheckUnits instead of ValidateQuantities, since ValidateQuantities is a subclass of CheckUnits. I think you'll only need to touch these lines of code.

As a side note, @namurphy maybe we should start looking into implementing Python stub files (.pyi). I don't know their workings in detail but it might be worth exploring.

@RAJAGOPALAN-GANGADHARAN With all my rambling over, yes have at it!!!

[namurphy]

Closed by #2346.