Make UXARRAY able to use CESM-CAM output (with UGRID)

[andrewgettelman]

A similar issue is in the CAM respository: https://github.com/ESCOMP/CAM/issues/468

Discussed with @erogluorhan and Steve Goldhaber. We would like to make CAM output UGRID compliant so UXARRAY can read the data without having to know exactly what dynamical core is being used, and so the needed information is provided by CAM without the user needing to work about it.

The initial idea seems to be providing the connection information to meet the UGRID standard in a separate file, with a global attribute in the CAM output files pointing to it. Additional information and metadata which is small could be added to every CAM file.

The connection information should be output by the model as it runs: not something the user has to find. Fine if an input file for the grid is copied into the output directories, reformatted if necessary.