ISMIP7 compliant output

[erwinlambert]

• Scalars to proper units, names, and metadata
• Gridded fields to individual files
• Complete gridded fields
• Scalars per IMBIE basin are required

This work follows up on Martim's implementations.

[erwinlambert]

ISMIP6 list

[erwinlambert]

Update: Output requirements aren't finalised yet, but they'll most likely include scalars per IMBIE basin. So best to compute those online. I'll upload the extended polygons to the data repo.