Fix week() calculation to start week 1 at days 1–7

[venom1204]

closes #2611

This PR fixes the long-standing bug in week() (#2611) by defining weeks sequentially: days 1–7 are week 1, days 8–14 week 2, etc.

hi @MichaelChirico @tdhock can you please have a review when you got time .
thanks.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.01%. Comparing base (a837dc9) to head (8f6d2b2).
⚠️ Report is 24 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #7274   +/-   ##
=======================================
  Coverage   99.01%   99.01%           
=======================================
  Files          81       81           
  Lines       15503    15503           
=======================================
  Hits        15351    15351           
  Misses        152      152           

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[github-actions]

• HEAD=issue____2611 slower P<0.001 for memrecycle regression fixed in #5463
• HEAD=issue____2611 slower P<0.001 for DT[by,verbose=TRUE] improved in #6296
  

Generated via commit 8f6d2b2

Download link for the artifact containing the test results: ↓ atime-results.zip

Task	Duration
R setup and installing dependencies	4 minutes and 56 seconds
Installing different package versions	9 minutes and 59 seconds
Running and plotting the test cases	2 minutes and 35 seconds

[tdhock]

lgtm