A greenhouse sensor takes per_day readings every day. They arrive as one long list: all of day 0's readings, then all of day 1's, and so on.
The sensor was switched off part-way through the last day, so the list may end with a few readings that don't make a full day.
Task: write daily_peaks(readings, per_day) and return the highest reading of each full day, as a plain list, in day order.
per_day is always at least 1.