Rename EQLHarmonic and EQLHarmonicSpherical classes into something more meaningful

[santisoler]

Description of the desired feature:

I would like to rename the equivalent layer/sources classes to something more meaningful.
Since both classes work with point sources rather an actual layer of mass distribution, I would be more prone to include "equivalent sources" in their name.
On the other hand, I don't think we should specify the "harmonic" nature of the fields these classes generate.
We don't have another type of equivalent sources interpolations, so I think we should not include the "harmonic" part until we really need someway to differentiate them from others (maybe we should differentiate future classes from these two instead).

I would like to add some backward compatibility on this, specially after the Harmonica tutorial at Transform21.
Maybe we could create two new child classes (whose names are EQLHarmonic and EQLHarmonicSpherical) that raises a FutureWarning after initialization.

Proposals:

• EquivalentSources and EquivalentSourcesSpherical
• EqSources and EqSourcesSpherical