Remove/redo SphericallySymmetricVolCondMEG.calculate_H_from_iaxial()

[espenhgn]

It makes little sense to include this method with the SphericallySymmetricVolCondMEG class, as the formula is mainly valid with infinite homogeneous volume conductors. It should remain in the InfiniteHomogeneousVolCondMEG class.

If this method should stay, it needs to be modified to use the correct formula from Sarvas J., Phys.Med. Biol., 1987, Vol. 32, No 1, 11-22 as implemented by the parent LFPykit.eegmegcalc.SphericallySymmetricVolCondMEG class.
It should also check that all source radii is less than the measurement site radius.

[espenhgn]

Accidentally pushed e441281 to master, which will raise an NotImplementedError when this method is called. Leaving this issue open, in case someone will implement this feature.

[github-actions]

This issue appears to be stale due to non-activity and will be closed automatically