Some adjustments to simplify ingestion of data

[mortenwh]

NetCDF files from MET uses ACDD as attribute convention. This is different from DIF in, e.g., that the unique id is called just id while in DIF it is entry_id. This update allows id if entry_id is not present.

In nansat, we generally just parse the json string representation of the gcmd keywords. A more readable format is, e.g., "Earth Remote Sensing Instruments > Active Remote Sensing > Imaging Radars > SAR > Synthetic Aperture Radar". This update also allows for that.

[mortenwh]

btw - if you have time at some point, we could meet to discuss how we can better align our efforts. I think it is time to look at that now :)