Allow reading in metadata partially

[TonioF]

Reading in all metadata of a data source may cost a lot of time, because in the background datasets need to be accessed. This can significantly slow down the retrieval of meta information if it is requested for many data sources. However, the datasets only need to be accessed for a relatively small portion of the information.
xcube-cci can be made a lot faster when this information is read only when needed and requested.

[forman]

See CCI-Tools/cate-app#120