Seasonal Date

Adds a seasonal date field type. Seasonal dates are used in some niche areas, most commonly in rainfall estimates, some information about the use of this measurement type can be found at http://www.fao.org/docrep/007/v9895e/v9895e00.htm#3.

Basically, seasonal dates are defined as grouped periods of 10 (dekads) or 5 (pentads) days. There are only ever 3 dekads and 6 pentads per month, the last of each being an irregular length. Because of this, it is difficult to make direct comparisons of these measurements to standard dates.