Support extra information for additional datasets

[zklaus]

Sometimes it is useful to provide extra information for the loading of data, particularly in the case of native model data, or observational or other data, that generally follows the established standards, but is not part of the big supported projects like CMIP, CORDEX, obs4MIPs.

To support this, it would be nice to have a facility that allows the passing of such extra information to both the data finder and possibly needed fixes.