Convert `Cube` -> "component", and vice versa

[ukmo-ccbunney]

A component in this context is any named cube part that is a DimensionalMetadata object, e.g. coord, coord_system, cell_measure, etc.

Converting components of a cube into cube objects (and vice versa) will facilitate the insertion of CF variables that were captured as "unloadable netCDF objects" into an existing cube (e.g. inserting a coord that failed to load correctly)

See for context: #6320 (comment)

A few considerations:

• The biggest issue is that there is the potential to lose information when converting from a component to a cube. E.g. when converting a coord to a cube, the original dimension(s) the coord mapped onto in the cube are lost. Similarly, any associated coordinate system is lost.
  • We could possibly retain this information by adding them as special "internal" attributes on the cube (see proposal in Namespaced "internal" attributes #6572). We would need to check that the merge code can deal ok with complex objects as attributes (should be ok - they are hashed for comparison operations)
  • Alternatively, operations that need to re-insert a component extracted from a cube will need access to the original cube to determine lost metadata.
• Should we have a separate metadata object to store the original component's metadata? This would need careful synchronisation with the cube's metadata in case common fields are changed (such as standard_name).