Add tests to compare plasmapy.formulary to PlasmaFormulary.jl?

[namurphy]

It's great to find out that there is a Julia package for containing a plasma formulary! 🎉

I'm wondering if it would be worhtwhile to add software tests in continuous integration to verify that PlasmaFormulary.jl and plasmapy.formulary both provide consistent results. This isn't necessarily needed for calculating straightforward quantities like the Alfvén speed, but would matter more for quantities like transport coefficients which are harder to calculate and verify. Setting up this would give us more confidence that both packages are providing correct results, and potentially help us figure out where the two packages make inconsistent default assumptions (like if the thermal speed refers to the root mean square, mean, or most probable velocity).

These tests could be hosted in either plasmapy.formulary (as a weekly test) or PlasmaFormulary.jl, but it would probably be redundant to include tests in both.

Thank you again for creating this package! 🌱