Show per day average for each tag in summary report.

By per day average in summary report, I mean that when we use summary command to filter tags, it should show per day average for those tags at bottom, just like total time is shown currently.
Because for a time tracking tool, it makes sense to have such a feature, which tells you about how time much you spend per day on specific things.

Start with the summary command and the code that currently prints total time for filtered tags. Trace how the report determines the relevant days and add a per-day average section at the bottom; done means summary output shows averages for the selected tags alongside the existing total.

Written by the indexing model from the issue text.