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 -
otherwiswe 'natural' values are used.

Authors:
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John A.R. Williams