Common Data Model reader requirements

This is an ongoing summary of what requirements the data readers should fulfill.

• The generated CDM objects (xr.Datasets) should have a single and known time definition
• If we choose to use the 'start_time', 'end_time' attributes, they should be correct for the whole data object.
• Some ECV netCDF files include spatial resolution information, this is nice to have.
• Usual coordinate names (lat,lon,time) should be predefined and the same for all CDM objects. E.g., aerosol ECV have 'longitude', 'latitude', such coordinates should be renamed upon reading the file.
• The generated CDM objects should have the same spatial definition. E.g., degrees.
• Maybe we can introduce some kind of a 'type' attribute for CDM objects, for ECVs that have a different spatial definition, e.g., shapefiles instead of a lat/lon raster.
• There should be a set of known Attributes that can be relied upon, e.g., long_name, standard_name, etc.