Add documentation and make doc strings clearer for compass/landice/mesh.py

[hollyhan]

The developers guide on the landice framework module compass/landice/mesh.py is currently missing documentation, which needs to be added.

Also, some of the doc strings in the code can be made more clear,
e.g., doc string for the argument section in the function get_dist_to_edge_and_GL

[trhille]

Thanks for flagging this, @hollyhan. I'll try to get to this soon.

[xylar]

@trhille, I'll add that @hollyhan and I were looking for details on which config options are needed from the provided section and how these values are used.

[trhille]

Okay, thanks. I'll make sure those are more clearly communicated.