Fix edge enforcement in 2d mesh generation

[corentin-prigent]

2D mesh generation is comprised of two steps:

• Insertion of all points given by the user, using Delaunay method, regardless of any user-provided edges.
• Verification that all user-provided edges are in the mesh. If some edges are missing, enforce their presence by performing swaps.

Swapping operator has been modified to prevent creation of flat triangles.

Some floating point comparisons have been modified in order to prevent false positives (comparison to zero has been replaced by comparison with MMG2D_EPSD, equal to 10^-10).

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 50.30%. Comparing base (c48f68d) to head (ce62c81).
⚠️ Report is 3 commits behind head on develop.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop     #323   +/-   ##
========================================
  Coverage    50.30%   50.30%           
========================================
  Files          177      177           
  Lines        47857    47836   -21     
  Branches     10355    10353    -2     
========================================
- Hits         24075    24065   -10     
+ Misses       16050    16041    -9     
+ Partials      7732     7730    -2     

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