Simplify the week calculations
Move writing the entry into class Week
Untangle the main program
Isolate the time conversion

This will be used from other places when we don't have a Tweet object
yet.
Minimize the number of Twitter API calls

Before, the API was called repeatedly for each week.

Optimise this process by looping over all tweets and picking the wanted
weeks. The `groupby()` function collects one week's tweets so they can be
passed to class `Week` as one batch.

Because the tweets arrive chronologically, the dates must also be
sorted, otherwise the termination and skip criteria wouldn't work.