code: private Matrix3d getM() {
  		if (M!=null){
  			return M;
  		}
  		M = new Matrix3d();
  		M.invert(getMInv());
  		return M;
  	}
docs: Returns the change of basis (orthonormal to crystal) transform matrix, that is
  M in the notation of Giacovazzo.
  Using the PDB convention (NCODE=1).
  The matrix is only calculated upon first call of this method, thereafter it is cached.
  See "Fundamentals of Crystallography" C. Giacovazzo, section 2.5
  @return