shorten daily averages at start of repository

[rinusser]

Currently the daily intake analysis averages the last n days for each day. At the start of a repository this results in lower averages, since e.g. on the first day there's only one tally for that day, but no prior data.

Change this behavior: days before the first Tally entry shouldn't be counted as empty: the first day's average should always equal that day's tally. The second day should average the first 2 days, and so on.

Missing days in between should still be counted as 0, like is currently being done.