subroutine addfld(name,long_name,units,
     |  f,dname1,lb1,ub1,dname2,lb2,ub2,idx)
!    
! Purpose: 
!   Save user-provided data f to secondary history field structure.
!     
!   Basically, the user provides a 2d array in any dimension combination
!   (geo or mag) containing all or part of the global domain, along with
!   the field name, long_name, and units. This data is saved in either
!   2d (lon,lat), or 3d (lon,lat,lev) data arrays (fsechist(i)%data) for
!   the final history variable at the current model time.
!     
!   If all 3 spatial dimensions are desired on the history, idx is the
!   index of the 3rd dimension (i.e., the dimension not specified by
!   dname1 and dname2). In this case, addfld is called from inside a
!   loop over the 3rd dimension. If only 2 spatial dimensions are desired
!   (lon,lat) then idx can be zero. However, only the levs dimension can
!   be excluded (i.e., idx can be zero only if saving a horizontal slice,
!   see table below)
!
!   This routine saves the data and field information in the derived type
!   fsechist(n) (see fields.F). The final variable on the history will be
!   either (lon,lat,lev,time), or (lon,lat,time), geo or mag (see subs
!   def_fsech and wrfsech in nchist.F).
!
! On Input:
!   dname1     Name (char string) of 1st dimension of f (see table below)
!   lb1,ub1    Lower and upper integer bounds of 1st dimension of f
!              (must be within full domain dimension sizes, see table below)
!   dname2     Name (char string) of 2nd dimension of f (see table below)
!   lb2,ub2    Lower and upper integer bounds of 2nd dimension of f
!              (must be within full domain dimension sizes, see table below)
!   f(lb1:ub1,lb2:ub2) 
!              Full or sub domain data to be saved for secondary history
!   idx        Index of 3rd dimension (or 0 if writing horizontal slice only)
!              (idx can be 0 only if providing f(lon,lat) or f(lat,lon))
!   name       Name of field (should be in namelist SECFLDS) (<= 16 chars)
!   long_name  Optional long name of the field (<= 80 chars)
!   units      Optional units of the field (<= 16 chars)
!
! On Output:
!   fsechist(n) is defined (derived type fields_sech, see fields.F). Data
!   component has been allocated, and data from f (full or sub-domain)
!   has been saved to fsechist(n)%data(lon,lat,lev)
!