This function calculates the fractional derivative of order ?d? for the
given function r(t). It is assumed that the vector ?r? contains the
samples of the continuous signal r(t) which we are going to calculate its
fractional derivative. ?h? is a constant and represents the sampling
period of r(t) (the time period between two samples). ?h? must be small
enough in the sense of Nyquist sampling theorem.
?y? is the result achieved by applying the fractional differentiation
operator on the input ?r?. This contains the samples of the real output
y(t) with the same sampling period used for ?r?.
It makes use of the Grünwald-Letnikov definition. The first element of
the vector "r", i.e. r(1), is always zero.