Add weeks

[bonjourmauko]

Fixes #966
Supersedes #979

[verbman]

I'm trying to call a variable with definition_period set to periods.DAY from a variable with a definition_period set to periods.WEEK.
I'm using the "add" argument in the format: persons("variable_name", period, "add")
It does not work and I get the error: "Unable to compute variable..... for period 2019-W05, as 2019-W05 and day are incompatible periods".

It does work if the called variable period is changed from periods.DAY to periods.WEEKDAY

[bonjourmauko]

@benjello I seem to have constrained week/day set inputs for coherence reasons (we can add and divide weeks and Monday to Sunday but not calendar days). What do you think? Also @MaxGhenis and @nikhilwoodruff. Would it make sense to make set inputs compatible between ISO weeks and calendar days for your use cases?

[bonjourmauko]

More generally I ask myself the question of weekdays' (for example 2022-W01-1) usefulness in practice.

[coveralls]

coverage: 73.942% (-0.7%) from 74.609%
when pulling fbce81f on add-weeks
into 169bef1 on fix-mypy-checks.