Bedmap2 restructure

[mdtanker]

• switched Bemap2 fetch to preprocess and save tifs as zarr files.
• fetch call only extracts individual files and places them into their own folders, instead of unzipping all the files at once.
• choose between eigen 64c geoid, gl04c geoid, or WGS84 ellipsoid reference models
• fill_na option (for surface and thickness), is performed on grid references to geoid, so when they're converted to the ellipsoid oceans should not equal 0, but equal the geoid height relative to the ellipsoid

Closes #119 and makes progress on #120.

[mdtanker]

Closed and replaced with #123. Accidently include commits from #118 here, so used git interactive rebase to remove those commits.