Add additional metadata to variables for UGRID/CF compliance

[ChrisBarker-NOAA]

assign_ugrid_topology() currently does (I think) everything that the subsetting code needs.

But there are other things that it would be nice to have, In particular, there's no addition of metadata to the data variables:

https://ugrid-conventions.github.io/ugrid-conventions/#data-defined-on-unstructured-meshes

which should have mesh and location attributes:

double Mesh2_waterlevel(time,nMesh2_face) ;
    Mesh2_waterlevel:standard_name = "sea_surface_height_above_geoid" ;
    Mesh2_waterlevel:units = "m" ;
    Mesh2_waterlevel:mesh = "Mesh2"
    Mesh2_waterlevel:location = "face" ;
    Mesh2_waterlevel:coordinates = "Mesh2_face_x Mesh2_face_y" ;

A "make fully ugrid compliant" method would be great.

[mpiannucci]

Going to keep this issue and close out #26 because i think they work towards the same thing