Use seeing from header in flux calibration

[julienguy]

• we should read the seeing values in the header now that we have them when computing the calibration vectors. This is either a change in desi_proc when calling desi_compute_fluxcalibration by adding --seeing-fwhm or a change to desi_compute_fluxcalibration itself (prefered option).

• The flux calibration vector should contain a header keyword that specify what seeing value has been used to compute the calibration. This is important for instance for the computation of TSNR.

[julienguy]

The ETC is now saving the header keywords described there https://github.com/desihub/desietc/blob/main/header.md to the fits image header. Things to do:

• Read those in the header when running the fluxcalibration.
• Convert them to a seeing FHWM computed with the fastfiberacceptance code (the seeing is then simply an internal variable used to get consistent fiberfrac values depending on the location of the sources in the focal plane and their intrinsic size) as done in the tsnr code: https://github.com/desihub/desispec/blob/e691ba421d1d1d8e426fae9d0ee76e724a1620c8/py/desispec/tsnr.py#L786
• Save this seeing value in the frame fits header so that we know what seeing value we have used for the flux calibration (this info is needed in particular for TSNR).
• Edit the TSNR code to read this new keyword and replace the default value of 1.1arcsec FWHM in https://github.com/desihub/desispec/blob/e691ba421d1d1d8e426fae9d0ee76e724a1620c8/py/desispec/tsnr.py#L853

[sbailey]

At the same time, also be robust to missing keywords or keywords that exist but are blank / None for robustness on past data and future hiccups.