To calculate an average you would most likely need to look at creating a custom metric for each of the metrics you are interested in.
Here an example to calculate Avg Daily Unique Visitors (which would then dynamically change based on dimensions/date ranges etc applied on your project).
If you wanted the above to become a Avg Unique Visitors p/week you could just multiply the value returned by 7.
Like this should work.
An note on the above though - where no data is captured for your metric (ie. Unique Visitors in the above example) the day count distinct wont consider that day in calculating the average.
hope this helps.