Remove GRASS script and update docs with `gdal_contour`

[alessandrofelder]

This PR

• removes the GRASS script
• replaces the GRASS docs with gdal_contour docs that serve the same purpose.

My understanding is that we used gdal_contour now instead of the GRASS script (that I, for one, didn't manage to get to work in the end - it did seem to work at some point for Okushiri - maybe it worked with an older of GRASS???), so the repo should reflect that this is now the preferred approach (mostly so we remember this when we come back to this in the future).

@Devaraj-G might want to confirm that I have got the above right!

Addresses #51 .

[Devaraj-G]

@alessandrofelder Since GRASS had issues with breaks in extracted contours (This can happen in gdal too), we have now moved to using the proximity contours for OceanMesh2D. Proximity is distance from the extracted contours, and proximity contours are smooth and continuous by nature. So yes, we can remove the GRASS part.