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.