improvements to turtle representation

[VladimirAlexiev]

Notes about http://vocab.nerc.ac.uk/standard_name/?_profile=nvs&_mediatype=text/turtle (which might be implemented in https://github.com/surroundaustralia/VocPrez-theme-nvs?)

• This file is quite slow to appear every time (>30s). Add some caching?
• Of these two prefixes, use only dct:

@prefix dc:   <http://purl.org/dc/terms/> .
@prefix dct:  <http://purl.org/dc/terms/> .

• You could redefine this below to use prefix dc: because that's most commonly used

@prefix dce:  <http://purl.org/dc/elements/1.1/> .

• dc:identifier vs dce:identifier: Is there need to use both (especially given that the former is a subproperty of the latter)?
• dc:date and pav:authoredOn "2018-02-12 16:53:18.0"should be"2018-02-12T16:53:18.0"^^xsd:dateTime`
• owl:deprecated "false" should be false or "false"^^xsd:boolean
• void:inDataset <http://vocab.nerc.ac.uk/.well-known/void> is wrong because that's a DatasetDescription not Dataset. It describes <https://vocab.nerc.ac.uk/> as Dataset, so use that one
• don't emit skos:altLabel when missing (currently emitted as "")
• skos:prefLabel "integral_wrt_time_of_radioactivity_concentration_of_222Fr_in_air"@en is hardly a name in English. Could you convert underscores to spaces?
• skos:note "accepted"@en: better use vs:term_status "stable" where vs: is http://www.w3.org/2003/06/sw-vocab-status/ns#