scilib/astutil.py: library-ify astrometric Monte Carlo

I also figured out why repeated computations were giving weird errors. It
turns out that Sesame has some issue where sometimes a repeated lookups of a
name give incomplete output after the first query. I switched over to a real
Simbad query, which seems like it should be more reliable.
scilib/astutil.py: define J2000 as an MJD