Loading of topographical data

[simonbyrne]

Load topographical data from a file and construct a z_surface field. This may require certain options:

• depends on #284
• should be consistent with interpolating polynomials
• preserve topographical peaks

[jakebolewski]

This also probably depends on being able to remap from a RLL grid to a cubed sphere (re-using the same infa) as all elevation products are distributed in RLL grids given some standard projection.