Add antarctic_ice_shelf_polys to downloaded data

[xylar]

Rationale

These data are quite useful for folks like me doing Antarctic research. We already include these data in the cartopy_offlinedata package on conda-forge.

Implications

Slightly more downloaded data for folks that don't work on Antarctica.

[xylar]

Testing

I made sure this works by running:

./tools/cartopy_feature_download.py -o ~/Desktop/cartopy_test gshhs physical cultural cultural-extra

As expected, I get the ne_10m_antarctic_ice_shelves_polys.* and ne_50m_antarctic_ice_shelves_polys.* files, along with the rest.

[xylar]

Here's an example usage:
https://github.com/MPAS-Dev/MPAS-Analysis/blob/develop/mpas_analysis/shared/plot/climatology_map.py#L778-L781
and a typical resulting image (from a very coarse resolution simulation):

[dopplershift]

Seems fine to me.

[xylar]

Thanks @dopplershift!