Add a method to compute polygon neighbours with linear barriers

* Added methods in PolygonEntity and PolygonGraph
* Added related tests

(closes OpenFLUID/openfluid#428)
Add a method to compute polygon neighbours with linear barriers

* Added a method for knowing the two PolygonEntity which share the same
Edge 
* Added compute neihbours methods in PolygonEntity and PolygonGraph
* Added related tests

(closes OpenFLUID/openfluid#428)