hera_librarian/utils.py: robustify metadata-gathering
hera_librarian/utils.py: robustify metadata-gathering

Although this needs to be thought out. Do we want to enforce that it must
always be possible to infer obsid and start_jd from any file stored in the
Librarian? Is the JD always in its name? Etc.

HERA-Team/librarian,HERA-Team/librarian,HERA-Team/librarian