Impose wavelength solution monotonically increasing

[jradavenport]

I think IRAF has this problem too...

Perhaps best thing to do is just a brute-force check after each fit, sweep through all wavelengths and make sure they increase. If not, throw a warning

[jradavenport]

have implemented a check for this. Going to be good enough for now I think.

still no solution to impose this as a condition, but could force it to be so in the updated automatic wavelength solution method (which will hopefully be the default user experience)!