Implement the fast iterative EQL interpolator

[santisoler]

Description of the desired feature

Would be nice to implement the fast iterative equivalent layer technique described on Siqueira, Oliveira Jr. and Barbosa (2017). It could be a fast alternative to the EQLHarmonic gridder from #78 that could be very useful when working with a very high number of data points.

The new class should inherit from verde.base.BaseGridder and follow its protocols. See the EQLHarmonic class defined on #78 to get inspiration.

Are you willing to help implement and maintain this feature? Yes

[santisoler]

@birocoles Would you like to tackle this down? And/or maybe get one of your students involved. I can help you implementing this!

[birocoles]

Hi @santisoler, I can handle this issue. I am currently trying to implement the horizontal components of point masses. After that I will address the present issue.

[santisoler]

That's great. Don't rush, feel free to start tackling it down whenever you are ready.