Feature: Add class level explorer to avoid unnecessary calls

[syphax-bouazzouni]

Context

Currently to get a concept children for example we do like this:

    @ontology = LinkedData::Client::Models::Ontology.find_by_acronym(params[:ontology]).first
    @concept = @ontology.explore.single_class({dispay: 'prefLabel'}, params[:id])
    @concept.children = @concept.explore.children

which is translated to 4 API calls

Getting: https://data.stageportal.lirmm.fr with {} (t: 0.22914239899910172s - cache: miss)
Getting: https://data.stageportal.lirmm.fr/ontologies/AGROVOC with {:include_views=>true} (t: 0.1970192330009013s - cache: miss)
Getting: https://data.stageportal.lirmm.fr/ontologies/AGROVOC/classes/http%3A%2F%2Faims.fao.org%2Faos%2Fagrovoc%2Fc_330834 with {:dispay=>"prefLabel", :include=>"prefLabel,definition,synonym,obsolete,hasChildren,inScheme,memberOf"} (t: 0.3808957980018022s - cache: miss)
Getting: https://data.stageportal.lirmm.fr/ontologies/AGROVOC/classes/http%3A%2F%2Faims.fao.org%2Faos%2Fagrovoc%2Fc_330834/children with {:pagesize=>750, :language=>"EN", :display=>"prefLabel,obsolete,hasChildren", :include=>"prefLabel,definition,synonym,obsolete,hasChildren,inScheme,memberOf"} (t: 2.8401889389988355s - cache: miss)

With the PR you can directly do

    cls = LinkedData::Client::Models::Ontology.explore('FMA')
                                              .classes(id)
                                              .children
                                              .get

which will be translated to the following calls

Getting: https://data.stageportal.lirmm.fr with {} (t: 0.22914239899910172s - cache: miss)
Getting: https://data.stageportal.lirmm.fr/ontologies/AGROVOC/classes/http%3A%2F%2Faims.fao.org%2Faos%2Fagrovoc%2Fc_330834/children with {:pagesize=>750, :language=>"EN", :display=>"prefLabel,obsolete,hasChildren"} (t: 2.3704248760041082s - cache: miss)

[syphax-bouazzouni]

Merged as needed in biodivportal, @Bilelkihal you can still review afterward.