Add facets to each page

[anitacaron]

Related to #51

At the moment it's returning mapping_justification and predicate_id that are part of the mapping model.

The mapping_set_title is part of the mapping set and need extra work to get them.

Facets object shows the list of values for each facet and the total number in total items:

"facets": {
    "mapping_justification": {
      "https://w3id.org/semapv/LexicalMatching": 185,
      "https://w3id.org/semapv/LogicalReasoning": 115,
      "https://w3id.org/semapv/ManualMappingCuration": 1092,
      "https://w3id.org/semapv/UnspecifiedMatching": 6740
    },
    "predicate": {
      "http://www.w3.org/2004/02/skos/core#closeMatch": 347,
      "http://www.w3.org/2004/02/skos/core#exactMatch": 665,
      "http://www.w3.org/2004/02/skos/core#narrowMatch": 155,
      "http://www.w3.org/2002/07/owl#equivalentClass": 6740,
      "http://www.w3.org/2004/02/skos/core#relatedMatch": 105,
      "http://www.w3.org/2004/02/skos/core#broadMatch": 120
    }
  }

[anitacaron]

@serjoshua, is this what you expect?

[pkalita-lbl]

This looks generally fine to me. Although if you find yourself doing a lot iterator grouping, counting, filtering, etc you might want to check out https://toolz.readthedocs.io/en/latest/index.html. For example your _create_facets could use https://toolz.readthedocs.io/en/latest/api.html#toolz.recipes.countby

[anitacaron]

Thank you, @pkalita-lbl. I'll try to use this countby method.

[serjoshua]

Looks good to me. Thanks!