Add magnetic force auxsolver

[jvwilliams23]

• Added functionality to compute magnetic force for systems with discontinuous permeabilities.
• Added hollow sphere levitation example (computes force on outer and inner surface).

[jvwilliams23]

FYI, some results on hollow sphere case. By changing -ref (refinement levels) we getting the following:

ref outer-integral inner-integral
0 359.0276 -0.002127189
1 373.2145 -0.0022614011

analytical outer surface integral is 372.88 N. Analytical inner surface integral is 0 N. So we have pretty good agreement.

I can test higher refinement levels if you want. I just ran these on my laptop for convenience.