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.