Removed Construct_second, source and target_point from KC.
(--> Construct_point_on_2.)
Added Construct_vertex for Segment, Iso*, Triangle and Tetrahedron.
Fixed Has_on_*, boundary only for full dimensional objects,
sphere predicates were missing.
Removed direction_from_* --> Construct_direction.
Added corresponding constructors to Direction.
Added Direction of Segment.
Added Construct_opposite_direction.
Added Compare_angle_with_x_axis_2 for Directions.
Removed Construct_min/max_point --> Construct_vertex.
Completed Equal functors.