PeriodStats - weekly stats

[leppott]

Is your feature request related to a problem? Please describe.
Add stats by week number.

Describe the solution you'd like
Use strftime(foo, format = "%U") to get week number then calculate stats.

Get by week and year-week.

Describe alternatives you've considered
strftime easiest way to get week number.

Additional context
Alternative Week Number versions in strftime.