Deviation in results for DOAS processing after `ifit_shift_updates` merge

[ubdbra001]

Results of processing the DOAS data deviate from past results after merge of PR #83.

These are small deviations, as illustrated in the image below (x column is pre-merge results and y column is post-merge):

So, unlikely to have a huge impact on emission results, but may be worth tracking down cause in future.

config includes:

use_light_dilution_spec: 0
LDF: 0

[twVolc]

I can't really work out why this would be - I've had a bit of a delve into ifit but there's nothing obvious there as it seems to treat an absence of LDF in exactly the same way as LDF=0: ifit.spectral_analysis.py Lines 533-544

 # Add effects of light dilution
        if 'LDF' in p and p['LDF'] != 0:

            # Calculate constant light dilution
            ldf_const = - np.log(1-p['LDF'])*(310**4)

            # Add wavelength dependancy to light dilution factor
            rayleigh_scale = self.model_grid**-4
            ldf = 1-np.exp(-ldf_const * rayleigh_scale)

        else:
            ldf = 0

Like you say, the difference is so small that I don't think we should worry about this at all, but it is a little odd...