laser wavelength must be monotonically increasing in LongitudinalProfileFromData

[rob-shalloo]

Have just come across a bug when using the LongitudinalProfileFromData profile in the spectral domain that the wavelength must be monotonically increasing. If the spectrum extends for example from 1000 nm to 500 nm rather than 500 nm to 1000 nm, then the function fails to produce something sensible.

The fix should be relatively straightforward by checking this condition and either adding an assert or flipping the array