Gallery example of gravity disturbance calculation (local)

[leouieda]

We have an example of global gravity disturbance calculation using spherical harmonic data. This is a global grid and is straight forward to calculate the geometric heights using gridded geoid values. It would be great to have a similar example using our land gravity data (#99). For this, we would have to interpolate the geoid values on the observation points (using verde.ScipyGridder with linear interpolation should be enough) so we can get geometric (ellipsoid) heights.