Test point in element

Introduces extension of the Element-interface by adding a method ```bool isPntInElement(GeoLib::Point const& pnt)``` that will return true if the point is located either inside or on the boundary of the element and false otherwise.
For line-elements it tests if the point is located on that line, for 2D elements the point needs to be coplanar with the element nodes and located inside the 2D element.

This introduces a little redundancy to the current Face-interface (i.e. Tri's and Quad's) where already a method ```isPntInside(pnt, eps)``` exists which basically does the same but allows to specify an additional epsilon. Currently, I have left this method as it is. Should I remove it?
no, the function returns a double.
basically the volume spanned by the parallelepiped defined by vectors ap, bp and cp is calculated (see my comment to DN for a more exhaustive explanation).
in ```isPntInTetrahedron()``` we only look at the sign of the result because if the parallelepiped using p has the same as sign as the one using d, both points are on the same side of the plane spanned by a,b,c.