ppDer(pp)

% dpp = ppDer(pp)
%
% Computes the time-derivative of piece-wise polynomial (PP) struct
%
% INPUTS:
% pp = a PP struct containing a trajectory of interest
%
% OUTPUTS:
% dpp = a new PP struct that is the time-derivative of pp
%
% NOTES:
% --> a pp struct is typically created by matlab functions such as
% spline, pchip, or pwch and evaluated using ppval.
% --> Call this function without arguments to run a test case
%