Simulation with double call to radiation for stratospheric (mostly volcanic) aerosols,
in order to calculate IRFs, with respect to "clean" case.

This option became available in the 6.1.11 version of IPSL-CM6. 

To activate the IRF calculation : 
   use flag ok_volcan in PARAM/config.def_* (here the _preind is used) :
	# Flag to yield double call with/without natural forcing (instead of with/without anthropic)
	ok_volcan=y

To activate the corresponding IRF output :
  See COMP/lmdz.card :
     Note the use of PARAM/*.xml_2RadCall files
       Key elements : 
	 in field*lmdz* : groups "fields_irf_aer_standard" and "fields_irf_aer_CMIPstyle"; 
	 in file*lmdz* : only the group "CMIPstyle" is activated, 
	    and it only contains the 8 "aerosol-radiation interaction" variables.
	   No aerosol-cloud interaction (4 variables), because stratosphere is very dry, no (very few) clouds.

      Also note in lmdz.card the post-processing (TS_MO) for the 8 aerosol IRF variables
       (with CMIPstyle irf* names instead of the native "tospswad, topswai…")

    The 6.1.11 version of PARAM/ping_LMDZ.xml file includes new "volcanic" variables, only needed for VolMIP.