Add ISMIP6 Antarctic ocean regions

[xylar]

These features are the ones used to select CMIP5/6 models for ocean forcing in the ISMIP6 project. They will be used for analysis in a paper that is in progress.

This merge also adds a creation script for these features.

[xylar]

@alicebarthel, this PR adds the features we discussed to geometric_features. It will require another step (which I will take care of) to be able to use them in MPAS-Analysis.

For now, I would like you to take a careful look at each of these 6 (just on GitHub is fine) and make sure they make sense:
https://github.com/MPAS-Dev/geometric_features/blob/8459c5f498ffaef3e7ca365a7aef407f09caf5a6/geometric_data/ocean/region/ISMIP6_Amery_Sector_Shelf/region.geojson

https://github.com/MPAS-Dev/geometric_features/blob/8459c5f498ffaef3e7ca365a7aef407f09caf5a6/geometric_data/ocean/region/ISMIP6_Amundsen_Sea_Shelf/region.geojson

https://github.com/MPAS-https://github.com/MPAS-Dev/geometric_features/blob/8459c5f498ffaef3e7ca365a7aef407f09caf5a6/geometric_data/ocean/region/ISMIP6_Totten_Region_Shelf/region.geojson

https://github.com/MPAS-Dev/geometric_features/blob/8459c5f498ffaef3e7ca365a7aef407f09caf5a6/geometric_data/ocean/region/ISMIP6_Weddell_Sea_Shelf/region.geojson

https://github.com/MPAS-Dev/geometric_features/blob/8459c5f498ffaef3e7ca365a7aef407f09caf5a6/geometric_data/ocean/region/ISMIP6_Ross_Sea/region.geojson

The Ross is divided into 2 halves because it crosses the date line (antemeridian). That's just a weird requirement of geojson files.

Here's a polar stereographic image for reference:

The other feedback I need from you is what zmin and zmax should be. Currently, they're -1000 and -200, but that's just because that happened to be what we used for the shelf in our previous Antarctic shelf regions.

[alicebarthel]

Xylar, the regions look good. Maybe some marginal differences but I reckon due to the plotting and topo, as the values shown in the raw geojson files correspond to the ones I have given. Please just clarify the continental shelf threshold when you can (it should be 1500m).

[xylar]

@alicebarthel, thanks for the review! I'm going to do a test to make sure this works in MPAS-Analysis before I merge this PR and make the release. It's a lot of work if I find out later that I made a mistake...

[xylar]

Merging based on testing in MPAS-Dev/MPAS-Analysis#777