naturaldelta rounds the value to the nearest unit that makes sense

[dangillet]

naturaldelta used to always round down. We want instead to round to the nearest unit that makes sense.

Fixes #174

Changes proposed in this pull request:

• naturaldelta rounds to the nearest value that makes sense
• correctly handle cases where we are rounding up, leading to a higher unit: 60 minutes -> one hour
• timedelta also round to the nearest value that makes sense

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.51%. Comparing base (50a413a) to head (e69ad3c).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #272   +/-   ##
=======================================
  Coverage   99.51%   99.51%           
=======================================
  Files          11       11           
  Lines         818      827    +9     
=======================================
+ Hits          814      823    +9     
  Misses          4        4           

Flag	Coverage Δ
macos-latest	97.82% <100.00%> (+0.02%)	⬆️
ubuntu-latest	97.82% <100.00%> (+0.02%)	⬆️
windows-latest	95.88% <100.00%> (+0.04%)	⬆️

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