Add sample station gravity data

[leouieda]

Description of the desired feature

We have a global gravity grid from spherical harmonics, which is great to show gravity corrections and global methods. It would be great to have some more local station data to show interpolations, localized inversion, etc.

There are a lot of datasets at https://www.ngdc.noaa.gov/mgg/gravity/

One that I find promising is this South African collection: https://www.ngdc.noaa.gov/mgg/gravity/1999/data/regional/africa/

It has different spatial coverages, which is great for showing and testing interpolation, and covers the Bushveld, which might have some interesting anomalies for modeling.

We should add this to our sample data. Probably will need to have a look first and see if there is anything in there or what the data quality is like. We might have to use a subset of the data if the files get too large. It will also probably require a geoid model since heights are relative to sea level. It would probably be good enough to interpolate from a global grid (#29).