The recently added `min_count` option to AverageAlgorithm.py leads to some
undesirable behavior when realtime data, with asynchronous inputs, are being
processed. By adding the ability to specify an interval over which `min_count`
is applied, some of this undesirable behavior can be mitigated.

In particular, if the `realtime` option is specified via the controller, and
`min_count` is defined, the minimum number of inputs will be allowed only for
time steps prior to `(UTCDateTime.now() - realtime)`; the full complement of
inputs will be required to calculate averages more recent than that. One
drawback is that if an input observatory goes offline for an extended period,
the Dst index will be calculated with a persistent lag `realtime` seconds long.

A user can always override this admittedly ad-hoc default behavior using the
`min_count_start` and `min_count_end` options.