Check that stray-light correction region of spectrum equals zero

[mattias42]

The pixel-region 50-200 is typically optically black, meaning that the pixels are covered
This region is used to perform a first-order stray-light correction. If the values here fall at zero then the offset of the spectrometer is too-low and the user should be given a warning on this as this will cause the spectra to give wrong evaluations.