Numeric 1st and 2nd order differentiation

This Perl package exports functions for performing numerical first
(Derivative1) and second Derivative2) order differentiation on vectors of
data. They both take references to two arrays containing the x and y
ordinates of the data and return an array of the 1st or 2nd derivative at
the given x ordinates. Derivative2 may optionally be given values to use
for the first dervivative at the start and end points of the data -
otherwise 'natural' values are used.