[b] = deriv1(a, nlin, npix, fil) Computes first derivative via FIR (1xn) filter Edge effects are suppressed and vector size is preserved Filter is applied in the npix direction only a = (nlin, npix) data array fil = array of filter coefficients, eg [-0.5 0.5] b = output (nlin, npix) data array Author: Peter Burns, 1 Oct. 2008 Copyright (c) 2007 Peter D. Burns