Support queries on atomic dataset domains

[bnlawrence]

We want to import "domain extents" while creating b-metadata. We'd do that by getting the range of the coordinates of each variable in an atomic dataset, and storing (separate) representations of each of the spatial bound box, and the the temporal bounding box (including calendar).

We also want "domain resolution".

How should we query this? We need to support simple use cases like:

• looking for global fields
• looking for X (cordex domain)
• looking for Y (named domain) (do we need to name domains we find as we load data from limited area models)?
• looking for a specific period
• looking for a specific resolution

(this last is a property of a domain, but not of a bounding box, suggests we need to name resolutions as we import them)

[bnlawrence]

There is now a cf-python ticket which would prepare some of the ground for this!