Support custom function spaces in LFRic (multi-data fields, different number of layers and data)

[TeranIvy]

Relevant LFRic documentation is in this LFRic wiki.

Update: The support will require a redesign of function space representation. See this new LFRic wiki.

Update, 21/10/2025: The design was discussed with STFC and the idea is to not have custom function spaces to capture the changes in nlayers and ndata as they are properties of the field and not function space continuity (the design of LFRic stores dofmaps in function space object). The current conclusion is to have additional metadata in the field argument types.

[TeranIvy]

• Metadata parsing support and related documentation. LFRic design wiki for metadata.
• Kernel arguments for fields on column (declarations, call argument list, stub argument list). LFRic design wiki for fields on column.
• Kernel arguments for fields on domain (declarations, call argument list, stub argument list). LFRic design wiki for fields on domain.
• Kernel arguments for operators (declarations, call argument list, stub argument list).
• Kernel argument lists for basis functions/quadrature (declarations, call argument list, stub argument list).

[mo-jdendy]

For LFRic #4511, scalar arguments and multiple function spaces are required within various inter-grid kernels. The functionality of this issue could be used to generalise inter-grid kernels.

The kernels in question map between function spaces of different element orders, which are on different meshes (making it an inter-grid kernel). The requirement for a scalar argument stems from PSyclone not passing the number of DoFs of the source space, as it is assumed that the function spaces of the source and target are identical. If PSyclone were to pass in ndf_source then I would not require scalar arguments, however issue #2504 would remain.