condition assignment draft 2

1. assign_cond_subcond.php selects a designated number of conditions
from among those not yet done by the participant, weighting
preferentially those with fewer completes in the db, then chooses a
subcondition for each and returns the lot.
2. simulation.html tests how well this works in practice.
3. submit_data.php is added to submit entire blocks of data instead of
rows.