adds an initialization value for the ratio in the solve function

[BenjaminPINEAU]

I suggest setting the initial value of the ratio to 0. In fact if verbose > 0 but there are 0 iterations, it results in the following error: exception = UndefVarError: ρk not defined in local scope

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 83.44%. Comparing base (e0f214d) to head (c90dad8).
⚠️ Report is 278 commits behind head on master.

Additional details and impacted files

@@             Coverage Diff             @@
##           master     #319       +/-   ##
===========================================
+ Coverage   61.53%   83.44%   +21.91%     
===========================================
  Files          11       12        +1     
  Lines        1292     1643      +351     
===========================================
+ Hits          795     1371      +576     
+ Misses        497      272      -225     

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[MaxenceGollier]

Thank you!