SES-1804 : Use standard language and datatype handling in RDFJSONParser
and RDFJSONWriter

To use this standard handling from RDFParserBase the parser needed to be
implemented as a non-static method, so extensively rearranging the
module for that.
SES-1804 : Fix NTriples and NQuads line skipping
SES-1804 : Convert asserts into reportError conditional checks