Add `hdrl_utils\airmass()` to `derive_photom`
See https://gitlab.astro-wise.org/micado/datareductionlibrarydesigndrs/-/merge_requests/56#note_23745 and #29 (closed)
Also add
\item \texttt{hdrl\_strehl\_compute()} by \hyperref[{dataitem:derivephotom}]{\FITS{derive\_photom}}
\item \texttt{hdrl\_dar\_compute()} by \hyperref[{dataitem:deriveastrom}]{\FITS{derive\_astrom}}, differential atmospheric refraction computation