Integration with Solr

[rth]

This is an issue that mirrors of the ElasticSearch integration #117 and aims to consider the integration of FreeDiscovery with Solr API

Multiple aspects of this question are possible,

• API design: it might be worthwhile to make the FreeDiscovery REST API follow closer the principles of the Solr API which would make easier for users to use both
• Solr integration within FreeDiscovery: FreeDiscovery methods that perform nearest-
  neighbor search in the semantic space (including LSI + 1-NN categorization) could be made to take as
  optional input an elastic search index, in order to combine search in the semantic space with regular
  search.
• FreeDiscovery integration within Solr: Solr has a system of plugins aiming to
  enhance the base functionality, and it might be possible to add a FreeDiscovery plugin (though the
  specifics remain to be determined).

This is a long term discussion ..