rational : operator that return "range" can be used in place where one call both begin and end functions that return iterator. It make it easier to use c++11 stil range loop. We already have the tool to construct a range from 2 iterators in xTool/xIteratorTools.h, and the range(int level) member already exist for xRegion. This commit add the operator for xMesh and xSubMeshManager, and use them in there respective implementation file.

other minor change :
xElementconstructor now take as in put a const xEntity * instead of an xEntity*. Rational:  xElment do not modify the Entity. easier to use in context when the

The following files are clang-formated

 xFEM/src/xElement.cc
 xFEM/src/xElement.h
 xFEM/src/xMesh.cc
 xFEM/src/xMesh.h
 xFEM/src/xSubMeshManager.cc
 xFEM/src/xSubMeshManager.h