Point masses gravity computation on geographic coordinates

[santisoler]

Description of the desired feature

The most simple geometry that generates gravitational fields are point masses.
Specially in spherical coordinates (or geodetic), where they represent one of the easier to implement forward models.
They can also be used to grid data through Equivalent Sources algorithms and to build tests for future functions.

Moreover, on #46 we propose to split the Tesseroid forward modelling into the discretization and the forward modelling itself through a GLQ. The latter can be addressed by the computation of the gravitational fields generated by point masses. So the point masses implementation would serve as basis for the Tesseroid forward modelling.

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