fixed issue with null datatypes
Default datatype in RDFLiteral to PLAIN_LITERAL.

The object relies on datatype being Nonnull in methods like hashcode, etc.
The constructor allowing null values is only used from test
Fix RioUtils to call RDFLiteral::isPlainLiteral instead of checking for datatype being null.
Merge branch 'version4' into owlapi-api-nullity

Resolve conflict in:
	impl/src/main/java/uk/ac/manchester/cs/owl/owlapi/OWLLiteralImpl.java