Design a better structure for the Antarctic regions bounding box polygon container

[weiji14]

Scattered across deepicedrain's various scripts is a 'regions' variable (also 'ice_boundaries', 'drainage_basins', etc) that holds a geographical bounding box and/or polygon which is needed for spatial subseting and plotting operations. However this is getting really messy/unwieldy and should be vendored off at some point to a package like regionmask.

This 'regions' container should hold Antarctic geographical boundaries (both a Polygon and BoundingBox) and have a 'placename' (e.g. "amundsen_sea_embayment") and 'full name' (e.g. "Amundsen Sea Embayment") attribute.

Related: d2b0199, 85b6ffa, #190