Revamp path naming in output directory hierarchy

[davideby]

The output directory paths have somewhat confusing names, using the producer_timestamp UNIX-time numeric value for each folder and e.g. apr23 (no year) as its parent. The former is confusing because we may use a (separate, distinct) timestamp for the random seed (see #28) but the two are not clearly distinguished anywhere. The latter is confusing because values from multiple years will accumulate in the same folder.