[strategies] Fix projected_point_ax+douglass_peucker compilation errors.

Add again operator< for projected_point_ax_result needed to compare the distance with 0 (explicitly converted into the same type).
Remove template parameter from projected_point_ax_less::operator().
Add tests for simplify() and detail::simplify_insert().
[strategies] Modify handling of 0-length segments in projected_point_ax.

For 0-length segments assume that the segment is perpendicular to the Pt->ProjPt vector.
In other words ATD = 0.