Compute distances in geodetic coordinates

Description of the desired feature

Currently Harmonica has functions to compute distances between points defined in Cartesian coordinates or in spherical geocentric coordinates. Would be nice if we add a function to compute Euclidean distances between points whose coordinates are defined in geodetic (ellipsoidal) coordinates (don’t confuse it with great circle distances). Worth noting that the geodetic coordinates depend on the reference ellipsoid, so we would need to pass a boule.Ellipsoid to the new function along with the coordinates of the two points. This may allow to compute gravitational and magnetic effects of point masses directly on geodetic coordinates without the need to convert them into spherical. Also it could enable us to build a separate equivalent layer class, a EQLHarmonicGeodetic for example, that interpolates harmonic data given in geodetic coordinates.

Some articles that may give insights of how to compute such distances are:

• Roussel et al. (2015). Complete gravity field of an ellipsoidal prism by Gauss–Legendre quadrature. doi: https://doi.org/10.1093/gji/ggv438
• Vajada et al. (2008). Global ellipsoid-referenced topographic, bathymetric and stripping corrections to gravity disturbance. doi: https://doi.org/10.1007/s11200-008-0003-5

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

[birocoles]

Ok @santisoler , I can tackle this. I will include more functions for converting coordinates in coordinates.py and then implement the distance in geodetic coordinates. I will create a first PR for this coordinate conversions.

[birocoles]

I can help