Allow single-span custom time aggregation

[rmartz]

A user may be interested in examining how a specific season or part of year changes over time. We should consider allowing the user to limit indicators to only evaluate for subsets of the year, either through choosing a band of time (Similar to the current band used for what years to graph) or with predefined seasons.

This may conflict with #233, as custom time aggregation currently cannot parse across a year boundary. Changes to the API to enable that may be necessary (azavea/climate-change-api#668?)