function data = data_diff_sg(data, order, porder, ncoeff)
SYNOPSIS
function data = data_diff_sg(data, order, porder, ncoeff)
DESCRIPTION
function data = data_diff_sg(data, order, porder, ncoeff)
Savitzky-Golay differentiation
order: differential order. Accepted values: 1 or 2
porder: polynomial order
ncoeff: number of filter coefficients. Must be off