define the RequiredTetrahedra keyword with binary tag 12, as in mmg3d

[mpotse]

This MR defines the RequiredTetrahedra keyword with binary tag 12. This tag was Reserved. The value is the one that I found in the mmg3d software. libMeshb had several types of Required entities, but this one was missing and I actually needed it.

BTW I wanted to propose the inclusion of several keywords for the distributed mesh format that parMmg is using, but 1) their binary tags are already used for other things in libMeshb, and 2) I think they need to be revised. So I'll come up with that later but I hope they can be included in version 8.

[LoicMarechal]

Bonjour,
vous êtes français d'après Adrien Loseille ;-)
Il faut éviter d'utiliser ces numéros de mots clefs réservés, car ils viennent justement de conflits de numéros qui sont arrivés dans le passé et j'ai dû complètement les désactiver pour éviter la confusion de plusieurs mots clefs qui utilisaient le même code.
Je suis disposé à rajouter le code RequiredTetrahedra mais en lui donnant le dernier numéro libre : 241.
Cordialement,
LM