add focc usage in the user guide and fix tests

add focc usage in the user guide

add has_bh column

add test for has_bh column

fix bug in accessing star table length

remove silly debug logic from get_stars

increase test coverage to 100%

fix math formatting

re-write confusing maglim argument

Formerly, the maglim argument was used in the SQL query, and future
calls would return this catalog. This can lead to unexpected behavior if
maglim_new > maglim_old, since maglim_old would be the only one to take
effect. The fix is to always download the full catalog and extract a
magnitude-limited catalog from there.