BUG: Fix GW_Face::GetEdgeNumber to return -1 if edge is not found

This commit fixes the following warning introduced in
3e42453 (ENH: Add SelectByPoints tool to DynamicModeler)

  /path/to/SurfaceToolbox/DynamicModeler/Logic/FastMarching/gw_core/GW_Face.inl: In member function ‘GW::GW_I32 GW::GW_Face::GetEdgeNumber(const GW::GW_Vertex&, const GW::GW_Vertex&)’:
  /path/to/SurfaceToolbox/DynamicModeler/Logic/FastMarching/gw_core/GW_Face.inl:359:9: warning: converting to non-pointer type ‘long int’ from NULL [-Wconversion-null]
    return NULL;
           ^

DynamicModeler/Logic/FastMarching/gw_core/GW_Face.inl

@@ -356,7 +356,7 @@ GW_I32 GW_Face::GetEdgeNumber( const GW_Vertex& Vert1, const GW_Vertex& Vert2 )
 				return (i+1)%3;
 		}
 	}
-	return NULL;
+  return -1;
 }