Add ms, µs and ns to time coordinates

[clarasb]

This PR adds ms, µs and ns as allowed units of time coordinates.

Closes #66.

Checklist (strike out non-applicable):

• Changes documented in CHANGES.md
• Related issue exists and is referred to in the PR description and CHANGES.md
• Added docstrings and API docs for any new/modified user-facing classes and functions
• Changes/features documented in docs/*
• Unit-tests adapted/added for changes/features
• Test coverage remains or increases (target 100%)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.46%. Comparing base (cbaf81a) to head (a7de3d0).

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #71   +/-   ##
=======================================
  Coverage   99.46%   99.46%           
=======================================
  Files          67       67           
  Lines        2810     2810           
  Branches      413      413           
=======================================
  Hits         2795     2795           
  Partials       15       15           

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