Creation of PolygonEdge attributes 

* Added PolygonGraph methods to manage PolygonEdge attribute Names
* Added PolygonEdge methods to manage attribute Values
* Added related tests
Fix of main return in boost tests
Merge branch 'develop' into develop-qt
Merge branch 'fix-boost-tests' of https://github.com/chataignerm/openfluid into chataignerm-fix-boost-tests