Add week_start parameter to floor() and ceil()

[krisfremen]

Pull Request Checklist

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• 🧪 Added tests for changed code.
• 🛠️ All tests pass when run locally (run tox or make test to find out!).
• 🧹 All linting checks pass when run locally (run tox -e lint or make lint to find out!).
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Description of Changes

Closes #1221

Add week_start(via kwargs to span) parameter to floor() and ceil()

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (403c29f) to head (b999fd9).
⚠️ Report is 19 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff            @@
##            master     #1222   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           10        10           
  Lines         2324      2339   +15     
  Branches       357       357           
=========================================
+ Hits          2324      2339   +15     

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