General equivalent layer solution

[leouieda]

Description of the desired feature

The equivalent layer (EQL) methods are great for interpolation, data integration, upward continuation, etc. Our implementations will be based on the Verde gridding classes. The most basic kind would be an EQL that uses 1/r as the basis function. This can be used with any kind of harmonic function but cannot integrate different fields (grav and gradients) and can't reduce to the pole. But it fits the Verde model well. We should implement this as a first step and then make more specialized gridders that can receive multiple fields as input.

The class should inherit from verde.base.BaseGridder and follow its protocols. It could live in a new package called harmonica/equivalent_layer/harmonic.py or something like that.