Improve FTS fitting

Adds changes from Andreas Lagg to improve FTS fitting:

Hi Manolo, Carl,

It’s been a long time since I last worked with GRIS, now even with the IFU.

When reducing the data, I found that the FTS fitting can be improved. The telluric line removal should be done not with the original FTS spectrum, but with the one convolved to the same spectral resolution as the GRIS data. This improves significantly the continuum fitting. It requires to add 2 lines to the fit2fts.pro routine (lines 92-94), attached the modified version from the ~mcvpro/idlpro directory on ulises4. Maybe this can be added to the default version of gris_v8

Another issue is related to my non-existing experience with the IFU: When reducing the data and then stitching it together using readily.pro, the continuum image does not really look too nice (see attached images, the seeing was quite okay, see the HIFI image). Am I doing something wrong? Is there a way to improve the stitched image?

We try to optimise the setup for the joint SolO GREGOR campaign currently running. The important dates for co-observations are Sep 16-18.

Thanks and best, Andi