Convert wavelength units when opening a file (or check units in DRAGONS)

[chris-simpson]

When astrodata reads a FITS file with a wavelength axis, the units (from CUNITj) are stored but not changed. However, DRAGONS assumes that the units of wavelength axes are nm, without checking.

So we have to implement one of two things:

1. Convert units at read time to nm
2. Wherever a wavelength is determined from the WCS, its units have to be checked and converted to nm if necessary

Clearly 1 is easiest; 2 is a pain (which is why we didn't do this originally).