Dataset processing fails with single wavelength

[IbkPLT]

Processing multiple chromatograms with less than 4 wavelengths (single wavelength detector) fails due to
"smooth_wl": max(len(self.wavelength()) // 20, 4), in line 298 of dataset.py.

A possible fix is "smooth_wl": max(len(self.wavelength()) // 20, 4) if len(self.wavelength()) >= 4 else None, to ensure that more than 3 wavelengths are present for the SG filter.