Issue: unique footprint IDs

[flx]

As discussed, for subsequent queries, VirGO re-uses the footprint ids from previous ones. Although this probably has not been stated clearly enough in the footprint note, it seems as if the SIAP/SSAP should only be required to provide unique footprint IDs WITHIN one VOTable. In order to be sure that the correct footprint is used, as of now, services need to provide globally unique footprint IDs which is technically not easily possible.