Extended subset tests to test code that resolved the below issue.
Extended subset tests to test code that resolved the below issue.

ISSUE: The issue was lying in loading data sources with meshed (2D) lat/lon coordinates. Cate attempts to convert these into
1D lat/lons, but requires coordinates with a uniform diff along the a lat/lon coordinate axis (because it is a grid).
The Seaice data does not conform to this requirement which caused cate to throw a cryptic Exception.

FIX: Cate refuses now loading such a data source throwing a ValidationError

CCI-Tools/ect-core,CCI-Tools/cate-core,CCI-Tools/cate-core