L6 : OPTIONAL argument position
********************************

Optional arguments to a routine shall be after non-optional ones.

To be avoided : 

.. code-block:: fortran
   :caption: OPTIONAL argument position

   SUBROUTINE OPT_ARG( ARG1, ARG2, ARG3)
   
   INTEGER(KIND=JPIM), INTENT(IN)          :: ARG1
   INTEGER(KIND=JPIM), INTENT(IN),OPTIONAL :: ARG2
   INTEGER(KIND=JPIM), INTENT(OUT)         :: ARG3