Set default stopping tolerances to epsilon^1/3

[BenjaminPINEAU]

Relax the stopping tolerances for the solvers (see #324).
Transition from $\sqrt{\varepsilon_M}$ to $\varepsilon_M^{1/3}$.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 83.85%. Comparing base (e0f214d) to head (7f5bb5d).
⚠️ Report is 281 commits behind head on master.

Additional details and impacted files

@@             Coverage Diff             @@
##           master     #326       +/-   ##
===========================================
+ Coverage   61.53%   83.85%   +22.31%     
===========================================
  Files          11       12        +1     
  Lines        1292     1641      +349     
===========================================
+ Hits          795     1376      +581     
+ Misses        497      265      -232     

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