Fix the procedure of deciding the term map when users don't specify
explicitly the term type (i.e., using rr:termType property).
Implement type conversion checking when rr:datatype is used in the
mapping document.
Merge branch 'feature/r2rml-support' into develop
Fix slight mistake on naming the datatype.
Implement toString() method to give the types a qualified-name.
Minor update for the exception message.