Proposition of change for the output formats

[jonquet]

As the MOD-API implementations shall return RDF data the proposition is to uddate the specification so that
TTL stays like this (format=ttl)
XML format has to be in fact => RDF/XML (format=rdfxml)
JSON format has to be in fact => JSON-LD (format=jsonld)
HTML => Every implementation chooses and returns the web page “the most appropriate” to answer the call