Add function to inspect the available data

[aurghs]

Would be nice to have a function that inspects the product directory, in order to expose to the user only the groups that are effectively available.

[corrado9999]

Is this still valid? We now check whether the annotation file is present and, if not, we remove the corresponding ssubswath.

[alexamici]

@corrado9999 yes this is in main since some time. @aurghs I close this one as well.