Fix SciPy cost logs

[NicolaCourtier]

Description

Fix the sign of the costs being stored in the SciPy optimiser logs.

Issue reference

Fixes #560

Review

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.17%. Comparing base (c8b00e6) to head (899ae6b).
Report is 106 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #561      +/-   ##
===========================================
+ Coverage    99.08%   99.17%   +0.08%     
===========================================
  Files           52       56       +4     
  Lines         3605     3988     +383     
===========================================
+ Hits          3572     3955     +383     
  Misses          33       33              

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[BradyPlanden]

LGTM - thanks @NicolaCourtier! Edit: please add a changelog entry :)