[xFEM] xBoundary.h/cc are moved to xFEM/src/attic
class xBoundary was only used as a parameter to some Mechanics::TreatmentOfNatEnv that was never actually used and which was passed around via a member of xData. It is nothing more than an early implementation of xSubMesh that was passed around via cut and past for a long time ... Therefore, xBoundary is now made obsolete and the files xBoundary.h/cc are moved to xFEM/src/attic. all its' use will be removed from xTest. Note : My first clang-format related bug. The file xDistanceNearestPointGenerator.h need to be included before xDistanceNearestPoint.h, otherwise some test case can't compile !!! indeed, it add some overload to cgal AABBPrimitive that needs to be visible for template instanciation that can take place via xDistanceNearestPoint.h. But clang-format messed around and I had to force the inclusion of xDistanceNearestPointGenerator.h before xDistanceNearestPoint.h, by including it at the beginning of xDistanceNearestPoint.h, and preventing clang-format to reorder the include by inserting some comment.