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
• WalkTEM example
• In-phase and Quadrature, with GEM-2 and DUALEM-842 examples
• Difference between magnetic dipole and loop sources

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:

• Change in the above Ward and Hohmann example area/(radius/2) to circumference; it is confusing as it is now. Clarify Ward and Hohmann circular loop #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.