Document better loops

[prisae]

Any kind of loops (including central loops as a special case) are possible in empymod, but not trivial, and I get a lot of questions. Hence, it should be better documented.

Note: The loop function only converts a magnetic dipole measuring the magnetic field H [A/m] (theoretical, there are no magnetic dipole equipments) to a magnetic coil measuring the magnetic flux B [T].

There are examples of it:

• Figures 4.7 and 4.8 from Ward and Hohmann: https://empymod.emsig.xyz/en/stable/gallery/reproducing/ward1988.html#ward-and-hohmann-1988-fig-4-7
• WalkTEM example: https://empymod.emsig.xyz/en/stable/gallery/tdomain/tem_walktem.html
• In-phase and Quadrature, with GEM-2 and DUALEM-842 examples: https://empymod.emsig.xyz/en/latest/gallery/fdomain/ipandqp.html

For a central loop one needs to use bipole or dipole, loop does not work as source and receiver would be at the same location.

TODOs:

• [x] Change in the above Ward and Hohmann example area/(radius/2) to circumference; it is confusing as it is now. #233
• [ ] Create specifically a central loop example.
  • [ ] Simplest case: symmetric central loop
  • [ ] Square central loop
  • [ ] Central square loop within bigger square loop
  • [ ] Loop where the receiver is in an arbitrary place
  • [ ] Arbitrarily shaped loop
  • [ ] ...
  • [ ] Also document dipole H, coil B, and in time domain dB/dt
• [ ] Bring a TEMfast example from @aignerlukas, https://github.com/aignerlukas/TEMIP_sensitivity
• [ ] Maybe create a central loop function in empymod.model, as it is often requested.