imation,
   'deriv',1        First-order derivative.
   'deriv',[1 1]    Second-order derivative.
   'deriv',2        For bivariate fits, partial deriv. wrt second variable.
   'deriv',[1 2]    Mixed second-order derivative.

 Density estimation is done on the log-scale. That is, the estimate
 is of g(x) = log(f(x)), where f(x) is the density.

 The relation between derivatives is therefore
     f'(x) = f(x)g'(x) = g'(x)exp(g(x)).
 To estimate f'(x), we must estimate g(x) and g'(x) (fit1 and fit2