iFit/func_eval

 Evaluate a function at the plotting bin centres of d3d object or array of d3d objects
 Syntax:
   >> wout = func_eval (win, func_handle, pars)
   >> wout = func_eval (win, func_handle, pars, 'all')

 Input:
 ======
   win         Dataset or array of datasets; the function will be evaluated
              at the bin centres along the plot axes

   func_handle Handle to the function to be evaluated at the bin centres
               Must have form:
                   y = my_function (x1,x2,x3,pars)

               or, more generally:
                   y = my_function (x1,x2,x3,pars,c1,c2,...)

               - x1,x2,x3  Arrays of x coordinates along each of the 3 dimensions
               - pars      Parameters needed by the function
               - c1,c2,... Any further arguments needed by the function e.g.
                          they could be the filenames of lookup tables for
                          resolution effects)

               e.g. y=gauss3d(x1,x2,x3,[ht,x0,sig1,sig2,sig3])

   pars        Arguments needed by the function. 
                - Most commonly just a numeric array of parameters
                - If a more general set of parameters is needed by the function, then
                  wrap as a cell array {pars, c1, c2, ...}

   'all'       [option] Requests that the calculated function be returned over
              the whole of the domain of the input dataset. If not given, then
              the function will be returned only at those points of the dataset
              that contain data.
               Applies only to input with no pixel information - it is ignored if
              full sqw object.

 Output:
 =======
   wout        Output object or array of objects 

 e.g.
   >> wout = func_eval (w, @gauss3d, [ht,x1_0,x2_0,x3_0,sig1,sig2,sig3]])

   where the function appears on the matlab path
           function y = gauss3d (x1, x2, x3, pars)
           y = (pars(1)/(sig*sqrt(2*pi))) * ...