Add always flag to add_on_update_callback

[shihab-dls]

closes #306

Adds an always flag to add_on_update_callback, and checks this flag when deciding what callback to call in update.
TODO: Check that this PR solves #133

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.69%. Comparing base (c43f71b) to head (470d6b2).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #311   +/-   ##
=======================================
  Coverage   90.69%   90.69%           
=======================================
  Files          70       70           
  Lines        2535     2537    +2     
=======================================
+ Hits         2299     2301    +2     
  Misses        236      236           

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