Better integrate magnetic source and receiver

[prisae]

emg3d can be used to model electric and magnetic sources with electric or magnetic receivers.

How to get the magnetic response from the electric field is shown here:

• 5a_Obtaining_the_magnetic_field.ipynb

How to calculate the electric and magnetic fields due to a magnetic source is shown in:

• 5c_Magnetic_source_using_el_loop.ipynb
• 5d_Magnetic_source_using_duality.ipynb

However, nicely implemented is the default, the electric field to an electric source.

Implement the other cases properly, with msrc and mrec parameters just like in empymod. Steps to take care of in doing so (not all are mandatory; just take magnetic loop sources and magnetic dipole receivers into account, should be fine):

• [ ] Expand mu_r from isotropic to tri-axial (otherwise duality will only work for isotropic resistivities). => https://github.com/empymod/emg3d/tree/triaxial-mu-r
• [x] Change current definitions of \eta, \mu_r to definitions \eta=V\sigma=V/\rho, \zeta=V/\mu_r.
• [x] Remove \epsilon-part (diffusive approximation).
• [ ] Include msrc (el. dipole or loop) and mrec (el. or magn. dipole) into emg3d.solver, hence there are four possible fields that can be returned; use duality (swap +/-; \eta/\zeta; src-dir).
• [ ] Add get_e_field.
• [x] Change time-convention from exp(-iwt) to exp(iwt) (not necessary, just to make it more alike empymod and avoid occasional confusion).

This, together with https://github.com/empymod/empymod/issues/47, will make empymod and emg3d very similar in behaviour and possibilities.

• [ ] Check documentation, docstrings and inline-comments to reflect the changes.
• [ ] Re-run benchmarks.
• [ ] Run all examples.

[prisae]

Notes

• msrc, mrec won't happen. Duality is not good. Themu_rparameter has first-order error (because of grad), compared to theres`-parameters which have second-order errors in its current implementation. Instead, one should create an electric loop to model a magnetic source.
• If we use electric loops than we get the electric field for a magnetic source. So for electric and magnetic sources we get the electric field, and get_e_field is not required.
• For the same limitation of first-order error for mu_r it will not be included tri-axial, unless there is once a real use-case coming up.

Closing this for now. If anything of this affects you, please re-open.

[prisae]

I think this is mostly addressed via the new survey and simulation classes and survey.Dipole.