Update iGrand to fix wavelength solution after some number of iterations

[bjfultn]

Update iGrand to use results of previous iterations as initial guesses for the next iGrand iteration.

[bjfultn]

Do we have a way to use the wavelength solution output in the .mod file as the initial wavelength file? I thought I remember @JayVB saying something like this at one point but I can't remember now. If we don't have something like this then I can write it in Python.

[bjfultn]

Try locking the wavelength solution after 3-5 iGrand iterations.

[bjfultn]

The issue is that the output wls files have lines for each observations, but the input to grand is expected to apply the same for all observations (NIM column == 0).