Adding code for locating the interval for a point.
Added a pass because I don't want to commit bum code.
There was actually *no* bad data. Never seen that before, but put a
guard in.
positions: use est scores instead of band filter.
Moved time filter into API.

The main program is installed as `rmg_time`. (Note that the command line
parameters don't yet conform to the standards of other runnables.) The
appropriate classes have been moved to a file called python/signal_filter.py.