If the input workspaces has spectra that have a wider wavelength range than the first
spectrum in the workspace the absorption correction calculation failed with an error:
MatrixWorkspace::yIndexOfX - X value is greater than the highest in the current range

I've fixed this and have also added a unit test that runs the calculation with an
input workspace featuring this kind of variation in the wavelength ranges.
In the existing tests the spectra all had the same x range because they were really
TOF spectra that had been relabelled wavelength