Adjust signature of magnetic_field method for magnetostatic classes

[namurphy]

Right now the MagneticDipole.magnetic_field method (from plasmapy.formulary.magnetostatics requires a position vector, p which makes it harder to do array operations. We could instead do something like having keyword-only parameters like x, y, & z...which would also let us have additional keyword-only parameters like r, z, and phi or r, theta, phi for cylindrical and circular coordinates, respectively.

[bucket420]

Hi I am currently working on this and have a couple of questions:

1. Do you want to keep the position vector p as an optional argument?
2. Is it better to use explicit keyword arguments or **kwargs?

[StanczakDominik]

1. Keeping p doesn't break backwards compatibility.
2. Explicit keyword arguments are easier to work with as a user - you can't get your editor to give you tips on what **kwargs handle, but with explicit keyword arguments that's easy.

Note however that #1688 exists! Still won't hurt to get this in for now, probably.