Necklace maps frontend throws exception in debug build

[Yvee1]

Running the frontend on the europe-population-necklace.json example in debug mode throws the following CGAL exception:

terminate called after throwing an instance of 'CGAL::Precondition_exception'
  what():  CGAL ERROR: precondition violation!
Expr: (m_traits.compare_y_at_x_2_object()(p, cv) == EQUAL) && compare_xy(cv.left(), p) == SMALLER && compare_xy(cv.right(), p) == LARGER
File: /usr/include/CGAL/Arr_segment_traits_2.h
Line: 609

The exception occurs in this line during computation of a feasible interval:
https://github.com/tue-alga/cartocrow/blob/b83a4cda8d64f486fdfd7a63214dd56bb76fb59e/cartocrow/necklace_map/feasible_interval/compute_feasible_interval.cpp#L64
During the approximate a new polygon is created; in debug mode CGAL checks whether the polygon is valid and throws an exception if it is not. Presumably the polygon was valid before calling approximate, so this may simply be some precision issue, but I'm not sure.

[Willem3141]

I think this is related to / the same issue as #23. (I'm totally unclear on why I gave that issue the title “RegionMap painter” as the stack trace doesn't include painter-related.)

It is unsafe to convert CGAL polygons / RegionMaps to inexact. I don't really know what to do about this. For necklace maps in particular we don't really need an arrangement in the first place...