failing to interface to planar ugrid

[mo-marqh]

this code should set up an XIOS conteaxt from an input mesh file

note code is XIOS3 only due to domain:: interfacing

but this fails with a
Program received signal SIGSEGV: Segmentation fault - invalid memory reference.

this appears to demonstrate limitations with XIOS3 handling of UGRID meshes where the coordinates are projection_x & projection_y

[NMannall]

Currently, this fails when trying to read coordinate values from the input file. To hit the error when building the mesh, I needed to change the units in planar_mesh.cdl to degrees_east and degrees_north respectively for mesh_node_x, mesh_node_y, mesh_face_x, and mesh_face_y. This is to help investigate the issue, and isn't a suggested change to the demonstration, which, in the long term, we want to use non-latitude and longitude coordinates.

I also had to fix a bug in XIOS3: src/io/netCdf_cf_constant.hpp line 24. The if statement checks XIOS_CF_Longitude_units_.empty() when it should be XIOS_CF_Latitude_units_.empty().