Added getters and setters for field variables.
Merge pull request #65 from zgzn/sbol2

Added getters and setters for field variables.
Renamed RefersTo to MapsTo.
Merge pull request #66 from zgzn/sbol2

Renamed RefersTo to MapsTo.
Merge branch 'sbol2' of https://github.com/SynBioDex/libSBOLj.git into sbol2
Working test serialization for Module