[GMSHPolygonTree] Removed method declarations with missing implementation.
[GMSH] Made docu more specific.
Substituted size_t with std::size_t.
[GMSHPolygonTree] Reduced indentation in insertPolyline().
Rename variable to make code more readable.
[PointVec] Documented private method uniqueInsert().
[GMSH] Moved inserting the Polyline to the end of the method.

Before inserting the new polyline, there should be some tests performed with the already existing polylines.
[GMSH] Reformating for readability.
[GMSH] Added GMSHPolygonTree::checkIntersectionsSegmentExistingPolylines().