Add weeks

[bonjourmauko]

Supersedes #979 Depends on #1139

TODO

• [x] Add baseline test coverage for periods
• [x] Add baseline test coverage for holders
• [x] Add DateUnit to replace string date units
• [x] Implement ISO Week Calendar
• [x] Implement week offsets
• [x] Implement week input dispatchs
• [x] Implement week input divides
• [x] Implement subperiods
• [ ] Split instant in two (ISO format and ISO calendar)
• [ ] Complete doc (code)
• [ ] Complete doc (mainstream)
• [ ] Simplify tests — or use a specific tool for brute-force testing
• [ ] Add typing
• [ ] Deprecate obsolete methods
• [x] Implement output dispatch
• [x] Implement output divide
• [ ] Adapt variables
• [ ] Adapt holders
• [ ] Adapt populations
• [x] Adapt simulations
• [ ] ?

[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 decreased (-2.7%) to 74.995% when pulling a24de40643d844fc12eec596abcad6ed9791bed1 on add-weeks into f7529a2f0da2447b2d7f960f48b5dc28c75ca198 on master.