Test a few "centre-point-angle" cases and add some
generic geom code for testing vectors and boxes are within
tolerance (since rounding often creeps in).
Much of the arc-checking code will be useful to other
construction methods (e.g. point-point-point).
There are expected failures for the bbox code when
the arc passes though, but does not end on, a quadrant point
(0, 90, 180, 270). This is due to a defective
implementation of SHAPE_ARC::BBox() that does not take
into account the quadrant points. This will be fixed
as a follow-up.
John Beard