Function get_nearest_time() behaves as expected now

A FilterAlgorith.py module function get_nearest_time() is supposed to return
the nearest *allowed* time for a given filter "step". This worked fine with
steps for second and minute data, whose allowed times are the tops of seconds
and minutes. However, for hourly (and daily, and any "average" type step),
things failed since the allowed times are the center of the interval (for
example, for hourly data, which is the average of all minute samples from
00 to 59, the the allowed time is 29:30).

One consequence was that if a user specified an interval with a start and
end time that did not encompass a given hour's allowed center time (e.g.,
start=AA:29:31, end=BB:29:29), the algorithm would still return a sample
for time BB:29:30. More generally, requests for average type data would
include an extra sample.

In this fixed version, if start=AA:29:31, and end=BB:29:29, nothing is
returned, as intended (and as always worked for non-average type steps).
Furthermoref start=AA:29:30, and end=BB:29:29, a sample for hour AA is
generated; if start=AA:29:31, and end=BB:29:30, a sample for hour BB is
generated, and if start=AA:29:30, and end=BB:29:30, samples for both AA
and BB are generated, all as intended.