filter images by certain times of year

[danbjoseph]

as part of assign_images filter images to only those from a certain times of year (e.g. only May 15 - Sep 15 in DC to get images only during a growing season). note that this is different from #76 which deals with dates in specific year(s). users should be able to define the periods as an optional flag when running the script, or by adding it the the config files created in #48

• should let the user define more than one period
• needs to handle a period that runs across the new year (e.g. November 15 to February 15)