;Adapt a model function meant for MPFIT to be used by LMFIT.
;The primary two differences are
; 1) LMFIT can't pass any extra parameters. These are stuffed into a common block
; 2) LMFIT requires derivatives. MPFIT functions do not normally calculate derivatives.
;    This function calculates numerical derivatives for the function.

function lmfitfun_adapt_function,x,a
  common lm_fit_adapt,function_name,args
  f=call_function(function_name,x,a,_extra=args)
  for i=0,n_elements(a)-1 do begin
    ap=a
    if abs(ap[i]) lt 1e-6 then ap[i]+=0.01e-6 else ap[i]*=1.001
    this_F=call_function(function_name,x,ap,_extra=args)
    this_dFda=(this_F-f)/(ap[i]-a[i])
    if n_elements(dFda) eq 0 then dFda=this_dFda else dFda=[[dFda],[this_dFda]]
  end
  return,[[f], [dFda]]
end