How should NDCube.resample handle extra_coords?

[DanRyanIrish]

Description

Initial implementation of NDCube.resample will ignore and drop NDCube.extra_coords. However, this must be resolved before we release this functionality. One option is to decide to always ignore extra_coords. However a preferable situation for users would be that extra_coords would be handled in some way.

What are the challenges for resample regarding extra_coords and what should be our policy?