Create a golden dataset for use in testing and development of phenology module

[davidorme]

The implementation of #347 is based on work by @zhouboya, who has provided datasets and example calculations based on a 10 year time series of half-hourly data from the DE-Gri FluxNET site. This data needs to be provided in the pyrealm_build_data source package in order to support development, regression testing and then example usage.