file for testing 2d types
* the begin of a README.txt
added trivial constructors for 2D types
* added conversion constructors from many types to 2D circles
* added elementary approx function
new files for intersection implementations
renamed Circle2D -> Disc2D
* added intersection test Disc2D <-> Disc2D
added Point <-> Disc2D intersection
* made Disc2D <-> Disc2D with Vec2-Info more robust