Description

closes #4863

In investgating the issue, we found that when saving a Mesh and converting them to MeshCoords had root a root problem in Mesh.filters(), where it was ultimately trying to guess the coord's axis, which relies on the standard_name.

This pull request addresses the issue by adding mesh_filters, which directly pulls the coordinates out of the mesh, without needing to consider their names. The construction of the mesh already gave us the information needed to do this. The original filters function calls mesh_filters before sending it to the metadata_filter like before, but removing the axis from the call because it is already accounted for.

To make the filter slightly more intelligent and to avoid the hasattr(self, "face_coords") line, _Mesh2DCoordinateManager implements its own mesh_filters to deal with face coordinates separately. It also gave the added benefit of raising a ValueError specific to the _Mesh1DCoordinateManager case, where it mentions that it only expects node and edge locations (not face). However, this has broken tests, so I will need to check if this causes any unexpected issues for users.

In the case for saving coordinates without a standard_name, we decided that naming it as "unknown" should be suffice, like what is done for cubes. In the case of using ncdump with the MWE:

ncdump -h ~/mesh_new.nc 
netcdf mesh_new {
dimensions:
        Mesh2d_node = 3 ;
        Mesh2d_face = 1 ;
        Mesh_2d_face_N_nodes = 3 ;
variables:
        int Mesh_2d ;
                Mesh_2d:cf_role = "mesh_topology" ;
                Mesh_2d:topology_dimension = 2 ;
                Mesh_2d:node_coordinates = "unknown unknown_0" ;
                Mesh_2d:face_coordinates = "unknown_1 unknown_2" ;
                Mesh_2d:face_node_connectivity = "mesh2d_face" ;
        int64 unknown(Mesh2d_node) ;
        int64 unknown_0(Mesh2d_node) ;
        int64 unknown_1(Mesh2d_face) ;
        int64 unknown_2(Mesh2d_face) ;
        int64 mesh2d_face(Mesh2d_face, Mesh_2d_face_N_nodes) ;
                mesh2d_face:cf_role = "face_node_connectivity" ;
                mesh2d_face:start_index = 0LL ;

// global attributes:
                :Conventions = "CF-1.7" ;
}

There may be some way to give a more meaningful name, but it will involve making assumptions which may not apply to everyone.

I have tried my hand at adding types as I go along, but most areas are beyond my knowledge. Ultimately, I would like to remove the Any types I added to make mypy happy, but will take more time for me to know how to best update them.

Checklist

• Included a What's New entry
• Incorporated type hints in any changed code
• Checked if tests need updating
• Checked if benchmarks need updating
• Checked if documentation needs updating
  • Docstrings (we love code examples!)
  • User Manual pages
• Checked if dependencies need updating
• Confirmed that the GitHub 'checks' on this PR are passing ✅
  (further reading)