Allow Heasarc.locate_data to also list encrypted data

The difference in HEASARC paths between public and encrypted data in NuSTAR is just a change of directory: /encrypted/ vs /obs/, I don't know if it is the same in all missions.
In any case, it would be good to have the option (keyword argument?) to also list these directories with locate_data. If then one has the key (=the rightful owner) they can also decrypt them.

Can it be done? Technically I'm sure yes, but politically?

Start at the Heasarc.locate_data entry point and trace how it builds paths for public NuSTAR data. Determine how an option could include encrypted directories across missions, then verify that the default behavior remains unchanged and that encrypted paths are listed when requested.

Written by the indexing model from the issue text.