discussion data normalizations: loops vs. dipoles

[lheagy]

cc @prisae, @dougoldenburg

[prisae]

Just comparing e-dipole-e-dipole, m-dipole-m-dipole, and m-loop-m-dipole:

https://nbviewer.jupyter.org/github/empymod/empymod-examples/blob/master/7b_Dipoles_and_Loops.ipynb

Some of that might be useful for this issue.