Implement a routine for calculating growing season from datetime and temperature series.

[davidorme]

The seasonal phenology dynamics requires estimates of conditions within the growing season of a site, so we need a routine to identify the growing season within a time series of observations across years.

This basically just takes a year or longer time series of temperature values and their timestamps and the growing season is "just" where the temperatures within years are above 0°C, but needs:

• Arguments to tweak treatment of cold snaps: how long a snap breaks a season?
• We might also need hemisphere to handle array inputs that include northern and southern summers and hence where the season should be measured around winter not summer equinox.
• This is going to differ for models on daily data versus subdaily data: is it mean daily temperature or mean daytime temperature.

[davidorme]

Relevant comments from this #348 (comment) by @Shirleycwj

It's a little bit more complicated when it comes to the growing season. I think we should open up the option of different methods to define growing season but use the temperature threshold as the simplest, default method. Using the temperature threshold only (above 0 degree), timestep up to monthly is actually acceptable due to temperature oscillation at the beginning and end of the growing season. Currently there are no absolute consensus on how growing season is defined (thermal or radiative or productive growing season, see Körner et al 2023 Ecology letters and Fu et al 2022 Global change biology ) or method to define growing season...so we could ask the user to provide DOY of start of the growning season (SOS) and end of growing season (EOS) to calculate growing season value for input variables. If the user works on phenology then they would have such information, otherwise they could use our simplified way to define productive growning season.