Fix GridapEmbedded#115

[zjwegert]

Fix the issue described in #115 in which the ACTIVE triangulation and GhostSkeleton were incorrect. The fix is as follows:

• ACTIVE Triangulation: we now use the IN/OUT tags from the CUT cells to construct the correct mask.
• GhostSkeleton: We use the aforementioned mask to check if cells from compute_bgcell_to_inoutcut are truly cut, if not we correct these for the purpose of building the GhostSkeleton.

Here are the result of the changes.
Before:

After:

Finally, conforming with Gridap, I've added an issues folder to the tests.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 76.92%. Comparing base (96f4d97) to head (18a8202).
⚠️ Report is 3 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #116      +/-   ##
==========================================
- Coverage   77.04%   76.92%   -0.13%     
==========================================
  Files          31       31              
  Lines        3603     3614      +11     
==========================================
+ Hits         2776     2780       +4     
- Misses        827      834       +7     

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