removed check_degenerate_face()

works with CGAL handling the non-simple rings
projected_plane_2 seems to be buggy, check with cube18.poly…
working on improved projected-plane(), doesn't work yet
working now

but case when a degenerate line is parallel with a plane wouldn't work
changed all to projection_plane_range()

and it seems to work all the time. More tests needed though
new projection_plane() function

removed the one with range and range_2 with Martin Newell's algorithm
since it requires a simple polygon as input
finished formatting the description of the "unit tests" solids in Markdown
Merge pull request #4 from tudelft-gist/degenerate2d

Degenerate2d