Klimatfabriken: *decision* on growing season start/end/length

We need a decision on whether these indices can be implemented to meed the Climate Factory timeline, i.e. ready for production by late March (provisional, can be changed either way). Note that earlier attempts (related to #77 (closed), #78 (closed)) were over-complicating things by including a user-specified delay period, which is relevant for some other application-specific indices that should be delat with in a later issue.