Ensure that required triangles are preserved

[laetitia-m]

This PR

1. Fix a bug introduced by PR Fix tags on parallel interface when in LS discretization mode - for ParMmg #239
2. Improve comments

Explanations of 1.
A bug was introduced by PR #239: triangles required by the user in the input mesh were not preserved anymore.

Indeed, in PR #239, if the face was not MG_PARALLEL, then the tags MG_REQ+MG_SURF were removed, while only MG_SURF should have been removed if the triangle was initially required by the user.

Now, i.e. in this PR, the tag MG_REQ of edges is removed only if the edge is MG_PARALLEL and MG_SURF.
By convention in ParMmg (see tag_pmmg.c in ParMmg), if an entity is

• parallel + not required: the tags are MG_PARBDY+MG_NOSURF+MG_REQ;
• parallel + truly required by the user: the tags are MG_PARBDY+MG_REQ;

so we remove the tags MG_NOSURF and MG_REQ only if the edge is effectively MG_PARALLEL and identified as MG_NOSURF.

[codecov]

Codecov Report

Attention: Patch coverage is 20.00000% with 4 lines in your changes missing coverage. Please review.

Project coverage is 50.45%. Comparing base (9e391f3) to head (3bcfeaa).
Report is 16 commits behind head on develop.

Files	Patch %	Lines
src/mmg3d/hash_3d.c	20.00%	2 Missing and 2 partials ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #274      +/-   ##
===========================================
- Coverage    50.47%   50.45%   -0.02%     
===========================================
  Files          174      176       +2     
  Lines        47158    47202      +44     
  Branches     10259    10263       +4     
===========================================
+ Hits         23802    23818      +16     
- Misses       15633    15652      +19     
- Partials      7723     7732       +9     

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[Algiane]

It seems OK.
I think that I can merge it as soon as we've checked that the ParMmg tests pass.

[Algiane]

Thanks!