Fix up internal multiplication by units

[dopplershift]

Description Of Changes

This cleans up multiplication by units internally within MetPy by eliminating all places where we multiply arrays by units on the right. This is motivated by a user-reported issue with get_layer and masked arrays (Unidata e-support GSX-461780).

In order to catch all of these and ensure that we don't add more, I have created a custom flake8 plugin for MetPy and added it to CI, as suggested in #1818. Right now this just checks for multiplication by units on the right, but it should make it easier to add additional similar checks in the future. The plugin allows for the "normal" multiplication when using constants (or nan like np.nan).

Checklist

• Closes Custom linting tooling #1818
• Tests added
• Fully documented

[dopplershift]

After some performance testing by @dcamron :

we're going to ban all uses of multiplication by units in the library code. Not only are they fraught with compatibility problems, they're universally slower. The flake8 plugin will catch all instances and suggest using Quantity instead. The code in tests/ will be allowed to stay as is because the performance isn't as important, and quite frankly I don't feel like changing ~730 of these occurrences manually.

[jthielen]

...must resist urge to figure out where the bottleneck is...

In any case, this sounds like a good change to me. Only concern is that a * units('whatever') -> units.Quantity(a, 'whatever') should not be promoted as a universal suggestion/rule, as doing this with upcast types (e.g., xarray DataArrays) or Quantities themselves (xref hgrecco/pint#1231) will break things terribly. Using the constructor directly definitely seems like the best option for construction when a is a downcast type.

[dopplershift]

@jthielen If I had to guess, there's a bunch of dispatch taking place in *, figuring out the proper final type and who's handling the multiplication, whereas Quantity() just directly stores whatever you pass as the underlying data. Good point about the other types, that reinforces our thoughts that we shouldn't update examples & tutorials.