Ensure that required triangles are preserved

[laetitia-m]

This PR

1. Fix a bug introduced by PR #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[bot]]

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 :warning:

Additional details and impacted files

```diff @@ 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]

Thanks!