Add utility function to convert a timestamp to a normalized minute time slot, e.g. every 10 minutes.
Add finder functions for datum that doesn't use multiple queries (i.e. replacement for solaragg.find_datum_samples_for_time_slot).
Fix passing of time slot seconds to solaragg.minute_time_slot()
Tidy up loose ends; add SQL function to return reporting-ready minute-aggregate data.
Use new solaragg.find_agg_datum_minute() function for minute-level aggregation.