Suggested RDF Turtle representation improvements

[VladimirAlexiev]

Based on the canonic example https://pleiades.stoa.org/places/579885/turtle

• don't emit blank nodes as values of cito:citesAsEvidence, cito:citesAsRelated, cito:citesForInformation!
  There's a different number for each, which suggests you got data but the conversion has somehow skipped it)
• don't emit empty strings like
  dcterms:bibliographicCitation "", "Wikipedia (English) Athens"
• add and use prefix for https://pleiades.stoa.org/places/vocab#
• use prefixes for foaf, skos
• add xsd:date datatype here:
  dcterms:modified "2016-07-13T14:23:21-04:00"^^xsd:date
• use dc: instead of dct: in dcterms:coverage "Athina/Athens"
  because dct:coverage is an object property with range dct:LocationPeriodOrJurisdiction.
• use proper xsd:gYear literal for pleiades:start_date, pleiades:end_date, eg "-0330"^^xsd:gYear (the year must have at least 4 digits)
• this reflexive link is useless, so remove it
  <https://pleiades.stoa.org/places/579885> owl:sameAs <http://pleiades.stoa.org/places/579885>
• is it really a good idea to use machine-tags in dcterms:subject (eg "dare:ancient=1")?
  I thought dcterms:subject is intended for human-readable info.
  In any case, use dc: because dcterms:subject "This term is intended to be used with non-literal values"
• wasDerivedFrom is intended for structured resources. So don't use it for things like

  prov:wasDerivedFrom [ rdfs:label "Barrington Atlas: BAtlas 59 B3 Athenae" ]
  prov:wasDerivedFrom [ rdfs:label "Pleiades" ]

  Better use dc:source
• You use one ontology for pref label, and another for alt labels. Better use skos:prefLabel for uniformity

   dcterms:title "Athenae";
   skos:altLabel "Athen"@de,
        "Athenae"@la,
        "Athens"@en,
        "Αθήνα"@el,
        "Ἀθῆναι"@grc;