Clean up callback conditions

[efaulhaber]

As discussed with @ranocha, the condition integrator.stats.naccept == 0 && integrator.iter > 0 is an historical remnant that has just been copied around without really thinking about it.

The callbacks are only triggered for accepted steps, so this condition is never true. The callback will only be initially triggered once with naccept == iter == 0 and then always with iter >= naccept > 0.

[github-actions]

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 89.53%. Comparing base (5359525) to head (7d64acc).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1960      +/-   ##
==========================================
- Coverage   96.11%   89.53%   -6.58%     
==========================================
  Files         460      460              
  Lines       36926    36915      -11     
==========================================
- Hits        35490    33049    -2441     
- Misses       1436     3866    +2430     

Flag	Coverage Δ
unittests	89.53% <100.00%> (-6.58%)	⬇️

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[ranocha]

Thanks!