Every grid variable should be an actual numpy array

[ChrisBarker-NOAA]

Example:

When loading and SGRID via the "conforming" path -- it keeps some (all?) of the grid variables as netcdf variables, rather than numpy arrays.

In PyGNOME, we sometimes expect them to be numpy arrays -- e.g. call .sum() on them.

I think node_lon and node_lat are the issues at hand, but we should check all.