Add support for remapping from MPAS vertices

[xylar]

This merge adds support for remapping from MPAS variables at vertices to comparison grids.

To make sure mapping files are unique, _vertices is appended to the name of the MPAS mesh if it maps from vertices instead of cells. We will eventually want to cache these new mapping files just like we do for the cell versions and compute them in compass with new meshes along with the existing mapping files.

To remap variables on vertices, you need to create a RemapMpasClimatologySubtask object with vertices=True as a parameter.

[xylar]

@akturner, this worked for me when I switched the barotropic streamfunction to be computed on vertices, rather than cells, see:
xylar@5f6ea2f

If you rebase your branch onto this one, you should be able to just add vertices=True when you call the constructor for RemapMpasClimatologySubtask.

[anirban89]

It works for me

[xylar]

Testing

We have rebased #898 onto this branch and are using the barotropic streamfunction computed at vertices for all plots. Everything seems to be working well there.

[xylar]

@anirban89, thanks very much for reviewing this!