add a key in LIB: for Sterheimer correction activation or not

[Ahmed-Naceur]

Actually, the STERNCORR logical variable is added in LIBEST.fto activate manually Sterheimer correction, as follows:

*----
*  APPLY STERNHEIMER DENSITY CORRECTION
*----
      STERNCORR=.TRUE.
      IF (STERNCORR) THEN 
         ALLOCATE(ISONRF(NBISO),ENER(NGROUP+1))
         CALL LCMGTC(IPLIB,'ISOTOPERNAME',12,NBISO,ISONRF)
         CALL LCMGET(IPLIB,'ENERGY',ENER)
         CALL LCMGET(IPLIB,'MIXTUREGAS',KGAS)
         CALL LIBSDC(NBMIX,NGROUP,NBISO,ISONRF,MIX,DEN,MASK,ENER,KGAS,
     1   ESTOP(1,1,1))
         DEALLOCATE(ENER,ISONRF)
      ENDIF

In the future add a key for user STERNCORR control.

The default value should be Sterheimer correction activation.