function data_window(data, xrange, xwindowinterval) Kills smoothly values above and below xrange. The transition zone will have xwindowinterval length the x* values are relative to data.x. They are converted to indexes. Applies a window to the coefficients to have smooth transition between their current values and zero. I chose the sigmoid function 1/(1+e^(k*x)) because it is easy to use This function stabilizes at around k*x = +-6 I want this interval to be stretched to wninterval = 30 wavenumbers (my first attempt, may be different below), because my region of interest is 1770-940 and I know that 1800-900 is still ok.