Set default stopping tolerances to epsilon^1/3

[BenjaminPINEAU]

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

[MaxenceGollier]

Thank you @BenjaminPINEAU!

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 83.85%. Comparing base (e0f214d) to head (5eedfc9).
⚠️ Report is 283 commits behind head on master.

Additional details and impacted files

@@             Coverage Diff             @@
##           master     #330       +/-   ##
===========================================
+ Coverage   61.53%   83.85%   +22.31%     
===========================================
  Files          11       12        +1     
  Lines        1292     1641      +349     
===========================================
+ Hits          795     1376      +581     
+ Misses        497      265      -232     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[dpo]

Thank you.