[RF] Refactor RooFitHS3 to solve various bugs and improve usability

The first implementation of RooFitH3 had a number of shortcomings, which this
PR addresses.

In detail:

  * It is now possible to read JSON files independent of the ordering
  * A priority mechanism has been implemented for importers and exporters
  * Duplicate and dead code has been removed
  * Many small bugs have been fixed
  * The JSONInterface has been made public and moved from Detail to
    Experimental, so users can write their own importers & exporters
  * The two unit tests have been fixed

Co-authored-by: Nicolas Morange <nicolas.morange@cern.ch>
Co-authored-by: Jonas Rembser <jonas.rembser@cern.ch>