Allow vector interpolation methods to pack and unpack vector fields.

[odlomax]

Is your feature request related to a problem? Please describe.

Vector field components in NWP systems are often stored as pseudo-scalar fields. It would be useful if vector-based interpolation methods (e.g., "spherical-vector") could pack these components into higher rank vector fields, then unpack them before returning from the execute operation.

Describe the solution you'd like

Ideally, I think there should be a configurable tool that can do the packing and unpacking. This could live in the atlas::util namespace. It could then be used by the interpolation method implementation. Configuration could be supplied via either:

• The configuration given to the Interpolation constructor.
• The source or target field set metadata at the execute stage.

Of the two options, I think the first is cleaner.

Edit: I've found a neat solution by setting "vector field name" metadata in the component fields. See #218.

Describe alternatives you've considered

This could be implemented outside of Atlas, as an interpolation pre- and post-processing step. However, it does require some knowledge of Atlas arcana in order to properly handle the field data structures. I imagine there are also others who could benefit from this functionality.

Additional context

No response

Organisation

Met Office

[odlomax]

All PRs merged. Closing this issue.