Add support for WEEK/DAY period

[bonjourmauko]

New features

This is a test-driven attempt to capitalise on #966 from @nikhilwoodruff, to add support for weeks in OpenFisca.

The main difference is that this draft makes a distinction between days and weekdays.

In doing so, we can treat both ISO dates and ISO week dates interchangeably.

Scope is yet more limited than your examples @nikhilwoodruff.

Right now, OpenFisca period model relies on the assumption that period fit well together. A YEAR is composed a 12 MONTHs .

My idea would be to rely on Instant for values, and let Period handle both date types, 2014-01-03 and 2014-W3-1.

We could either let the parser do that with the actual type string, or separate the two periods in different classes and let Period orchestrate/dispatch between the two as needed.

• If A's definition period is the week, can I calculate A for a year?

Yes, as long as we identify common years of 52W or leap years of 53W, or that we use the average of ~52.2 weeks per year.

• If so, what do I do with the weeks that are only partially included in the year?

TBD

• Can we have a generic answer to the previous question, or is it going to be country-specific?

TBD

Comments? @sandcha @guillett @Morendil @Br3nda @sarafalamaki @benjello et al.

[bonjourmauko]

Superseded by #1146