Add progress callback support for all remeshing modules

[kmarchais]

Adds an optional progress callback to mmg3d / mmg2d / mmgs that reports iteration progress and supports cooperative cancellation (return 0 to abort). When unset (the default), the only runtime cost is a NULL check on the existing iterative loops.

Incidental change worth flagging: MMG5_adpdel previously discarded the return value of MMG5_optbad. With this PR a cancellation can originate inside optbad, so the return value is now checked and propagated.

Happy to add a libexamples/<module>/progress_callback/ style example as a follow-up if useful.

[CLAassistant]

All committers have signed the CLA.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 50.24%. Comparing base (ae7d305) to head (c58ae35).

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #324      +/-   ##
===========================================
- Coverage    50.31%   50.24%   -0.07%     
===========================================
  Files          177      177              
  Lines        47831    47913      +82     
  Branches     10352    10372      +20     
===========================================
+ Hits         24065    24073       +8     
- Misses       16037    16099      +62     
- Partials      7729     7741      +12     

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