Add support for MPAS grids

[huiwanpnnl]

E3SM is moving towards a unified "surface mesh" for its non-atmosphere components, e.g., ocean, land, river, sea ice, etc. Colleagues from some of the "surface" components have expressed the wish that QuickView supports the unified mesh.

• We have been provided with a sample data file from the MPAS ocean model, which also contains the connectivity information–see this URL.
• We have also been provided with this grid description. I find Tables 2.1-2.3 on page 4 and Figure 2.1 on page 5 very helpful.

@berkgeveci, could we start with adding support for variables with an nCells dimension? After that, we could meet with the surface model developers to see what support they would like to have for the vertex-based and edge-based variables. Thanks!