Add magnetostatics

[simbilod]

First step to adding magnetostatics

• Improve documentation, adding a new page for electrostatics / magnetostatics
• Add two great references to the citations
• Magnetic potential calculation
• Inductor example (partial, need improvement)

There are some minor issues highlighted in the notebook

• Sign error on the magnetic field calculated from the curl of the magnetic potential?
• Magnetic potential slightly different between analytical and FEM
• Magnetic field very different between analytical and FEM

I might be doing the curl wrong, and the boundaries might be messing up the magnetic vector potential.

FEM:

Analytical (cylindrical coordinates, mirrored about r-axis):