Several changes in bitia

Bitia no longer puts the date in the output filename; instead, it
names the files filename_prefix-0000.tif through
filename_prefix-XXXX.tif, where XXXX is files_to_produce-1 (the
files_to_keep parameter has been renamed to files_to_produce).

In the course of making these changes, some datetime-related helper
functions have been added to timeseries.py.