Replaced istice with istice_mec

The two are equivalent. istice is the result of
refactoring in CTSM: ESCOMP/CTSM@1fa503e

Author: kvrigor

src/clm5/biogeophys/SoilTemperatureMod.F90

@@ -719,7 +719,7 @@ subroutine SoilThermProp (bounds,  num_nolakec, filter_nolakec, &
                l = col%landunit(c)
 
                ! Select method over glacier land unit 
-               if (lun%itype(l) == istice) then
+               if (lun%itype(l) == istice_mec) then
                   select case (snow_thermal_cond_glc_method)
                   ! TODO, this code duplication isn't ideal and should likely be in it's own subroutine
                   case('Jordan1991')