Fixed freezing when source file for inject generator does not exist

* Added exception throwing when file does not exist

(closes OpenFLUID/openfluid#236)
Added polygons intersection method

* Added source code
* Added tests
Allow loading float values from rasters to PolygonGraph

* Modified code based on GDALFPolygonize method 
* Updated tests
Fix of non-critical misspellings in landr library

* Fixed mispellings in Exceptions
* Fixed bad english translations
Allow mean altitude calculation on LineStringGraph

* Added a LandRGraph virtual method
* Modified mean altitude calculation on PolygonGraph method
* Added mean altitude calculation on LineStringGraph method
* Uptaded related tests
Creation of PolygonEdge attributes 

* Added PolygonGraph methods to manage PolygonEdge attribute Names
* Added PolygonEdge methods to manage attribute Values
* Added related tests