returns true iff the three points p, q
and r form a left turn or if they are collinear and
the distance of $$q to $$p is
larger than the distance of $$r to $$p, where $$p is the point
passed to the object at construction.

Precondition:

$$p does not lie in the interior of the segment $$rq,
i.e. $$p is an extreme point with respect to $${p,q,r}.