Nake a start on derivig flux calibrartion
Normalize flat field correction to right edge

Previously we had been normalizing by the mean.
Using the right edge is better means we have to do less flux calibration.
Fit polynomial to each order for flux calibration
Do graph of fit for flux calibration

We mask out the lines as best we can - using a wider band for the
stronger lines, but the fit still isn't perfect.

This is mainly because a polynomial is not very appropriate.  I will
switch to a Gaussian plus polynomial.

Unfortunately, this means switching to lmfit.
Switch to lmfit for flux calibration fits

We now use a Gaussian plus a constant, and this works really well.