Realm extraction from file attributes

[charles-turner-1]

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

Follows from #481

Describe the feature you'd like

#478 allows us to extract the realm from file metadata using the OM3 builder. We want to extend this capability to all builders.

[joshuatorrance]

The core of this is already done I think.BaseBuilder.parse_ncfile attempts to get realm from the file for the _NCFileInfo object and then specific builders can do what they want with this info - they usually overwrite it with something gleaned from the filename since it's a new feature.

If there's a new builder for a dataset that has realm as a well behaved .nc file attribute then nothing needs to be done.

If we wanted to stop the existing builders from getting the realm from filenames then we'd first need to add 'realm' to their datasets' files' metadata attributes. A stepping stone towards this might be to get the existing builders to first check if realm has already been extracted from the file before trying to parse the filename.

Incidentally, one detail not accounted for at the moment is realm potentially having multiple values which is allowed by the CMIP6 spec, i.e. realm='atmosChem atmos'.