extend the file to dataset association agent to cover all sites in addition to EOS

[DomenicoGiordano]

The current implementation of the file to dataset association agent assumes that only XRootD EOS popularity is available.
Indeed the XRootD popularity DB includes now also data from other sites.
THe agent needs to be extended to cover also associations for files other than the EOS ones.

At the meantime it would be useful to evaluate a redesign of the association agent, based on different queries to phedex (and query frequencies)