The contouring algorithm incorrectly assumed that each subsequent
intersection point encountered while traversing the edges from
minimum to maximum value would not be less than the current clip
value. This is not true when a cell has scalars that do not
monotonously increase around the edges from the minimum to the
maximum.

This has been addressed by:
1) detecting a decreasing scalar value
2) determining the remaining polygon
3) searching the new minimum
4) re-start traversal from the new minimum

The bookkeeping of the polygon points, clockwise/counter-clockwise
iteration and generation of sub-polygons for each band has been
rewritten using std algorithms and iterators.