A garden sensor takes per_day temperature readings every day, always at the same times. Its log arrives as one long list of text, like "12.5" or "-999": all of day 0's readings, then all of day 1's, and so on. The list always holds whole days.
When the sensor loses its signal, it doesn't skip a reading. It writes the marker number missing instead.
Task: write slot_average(readings, per_day, slot, missing). slot says which reading of the day you want: 0 is the first reading of each day, 1 the second, and so on. Return the average of that slot's readings across all the days, leaving out any reading equal to missing. Round it to 2 decimal places.
If every reading in that slot is the marker, there's nothing to average. Return None.