Improvement of the function Angle2D in Attachment 1 (CXX)

This problem is reported on the Forum
(http://www.opencascade.org/org/forum/thread_17712/).

"I have been having problems with BRepAlgoAPI_Cut when I have two solids that
have overlapping faces and where one solid is not entirely inside the other.
Think of a small cube and a large cube that share one corner. OpenCascade
constantly crashed when trying to cut these.
In the end I have tracked it down to a failure in the Angle2D method of
BOP_WireSplitter. At the end of this function it creates a gp_Dir2d instance and
then crashes when it calls the Angle function."

The suggested (extended by Author) fix is attached.