Use `pint` to automate unit scaling

[LiamPattinson]

Rather than requiring the user to get the scaling factor and new units right, I'm wondering if it might be possible to make pint a required dependency and have pint work out the scaling for you, so you could just say rescale_coords("mm", coord_names) and it'll work regardless of whether the original units are in femtometers or parsecs. I'm pretty sure you can even get it to do things like temperature to keV if you use the right settings. Possibly an extension for later though!

Originally posted by @LiamPattinson in #71 (review)

[LiamPattinson]

I think something like this would do it:

import pint
ureg = pint.UnitRegistry()

def scale_factor(from: str, to: str) -> float:
    return (1.0 * ureg.Unit(from)).to(to).magnitude

Some clever error handling might be needed to report unrecognised units, plus it should probably work either with str or directly with pint types.