Update edge tags in splits

[coprigent]

This update changes how edge tags are initialized during pattern splits. Newly created edges in faces of original tetrahedra now receive tags from this face (except MG_REF tags). Tags of edges created inside original tetra are still set to zero.
The main purpose of this update is to ensure the validity of edge tags along the whole remeshing process (especially MG_BDY tags).
With this update, continuous integration tests provide altered outputs compared to previous versions of the code. This is due to the fact that, because edge tags are modified, more collapses are now allowed in the process. The following tests are in cause:

mmg/src/mmg3d/mmg3d1.c

Line 956 in b899536

if ( p0->tag > tag ) continue;

Before the update, the situation where a MG_BDY point would be attempted to be merged over a non-boundary edge would be refused. After the update, some collapses are now allowed because the corresponding edge is non longer non-boundary: it now has a MG_BDY tag, leading to a valid collapse.

[Algiane]

Thanks!