removed comment and added file xFEM/src/xDomain.h

[xfem] xDomain, zone_tag.

Previously, we had a raw aomd tag, zone-tag, created throught xDomainStringManager with the name "zone". It was use to set that a mesh entity was belonging to a "zone" with a id related to a zone_name throught xZone. It was set for example using xClassifyOn and It was used in particular by the xEntityFilter xAccept, xAcceptNot, xAcceptOnZoneId, xAcceptUpperAdjancencies And xGeomElem getZone() member function.

    * I replaced the usage of a raw aomd tag by a new class xDomain defined in xDomain.h, implemented using an xDataManagerAOMD.
    * xClassifyOn, xAccept, xAcceptNot, xAcceptUpperAdjancencies and  xGeomElem::getZone() are reimplemented using this xDomain.
    *xAcceptOnZoneId was not reimplemented since it was almost the same as  xAccept. Previous usage of xAcceptOnZoneId(int) can be safely replaced by xAccept(int)

    *xDomainStringManager.h and xDomainStringManager.cc that where previously only use to set up the aomd tag zone_tag are removed.
    *xDomainAttachableData.h that used to define an mAttachableData suittable to attach domain information is now useless and removed

    *Since it was previously stored as an attachData, it was automatically destroyed each time an mEntity was destroyed. To have the same effect, the xMesh::del(mEntity *e) have been updated to clear the xDomain from mention of e.

    *The header inclusion #include "xDomainAttachableData.h", "xDomainStringManager.h" are removed where needed in the library :
     - xCut/xLegacySimpleCut/src/xMeshCut.cc, xFEM/src/xEntityFilter.cc, xFEM/src/xEntityToEntity.cc, xFEM/src/xGeomElem.cc, xFEM/src/xIntegrationRule.cc, xFEM/src/xMesh.cc, xPhysics/src/xpThermic.cc

    all the following files are clang-formated :
       xCut/xLegacySimpleCut/src/xMeshCut.cc
       xFEM/src/xEntityFilter.cc
       xFEM/src/xEntityFilter.h
       xFEM/src/xEntityToEntity.cc
       xFEM/src/xEntityToEntity.h
       xFEM/src/xGeomElem.cc
       xFEM/src/xIntegrationRule.cc
       xFEM/src/xMesh.cc
       xPhysics/src/xpThermic.cc
 Changes to be committed:
	modified:   xCut/xLegacySimpleCut/src/xMeshCut.cc
	new file:   xFEM/src/xDomain.h
	deleted:    xFEM/src/xDomainAttachableData.h
	deleted:    xFEM/src/xDomainStringManager.cc
	deleted:    xFEM/src/xDomainStringManager.h
	modified:   xFEM/src/xEntityFilter.cc
	modified:   xFEM/src/xEntityFilter.h
	modified:   xFEM/src/xEntityToEntity.cc
	modified:   xFEM/src/xEntityToEntity.h
	modified:   xFEM/src/xGeomElem.cc
	modified:   xFEM/src/xIntegrationRule.cc
	modified:   xFEM/src/xMesh.cc
	modified:   xPhysics/src/xpThermic.cc