WebSearch: most popular field values optimisation

* Optimises get_most_popular_field_values().  Uses numpy.lexsort()
  when available.  Improves time spent in Python.  The bottleneck
  is now the database for searches giving more than 100000 results.
  (closes #1096)

* API change: get_most_popular_field_values() now returs
  list of tuples instead of tuple of tuples.  The commit amends
  callers in WebAuthorProfile, BibKnowledge accordingly.

Co-authored-by: Tibor Simko <tibor.simko@cern.ch>