Support ESMF Grids with 1D `elementConn`

[philipc2]

Proposed new feature or change:

From ESMF Docs https://earthsystemmodeling.org/docs/release/ESMF_8_1_1/ESMF_refdoc.pdf

If the grid contains some elements with large number of edges, using a 2D array for elementConn could take a lot of space. In that case, elementConn can be represented as a 1D array that stores the edges of all the elements continuously. When elementConn is a 1D array, the dimension maxNodePElement is no longer needed, instead, a new dimension variable connectionCount is required to define the size of elementConn. The value of connectionCount is the sum of all the values in numElementConn.

[philipc2]

@kmsampson

Pinging from our email discussion.

Would you happen to have a small ESMF grid that uses this 1D representation that we could use for testing?