fix voronoi plot

[SarahRo]

Description

The surface plot in energy_based_electrode_design.ipynb should be closer to the contour plot of the result.

Cause of the difference is that the cost is recomputed for plotting for the contour plot whereas the surface plot uses result.cost. result.cost is identical to logger.cost_convergence and hence, rather than storing the cost for the corresponding points result.x_model, it contains the best cost up to each point in the optimisation process. To fix this, result.cost now returns the same as logger.cost and result.cost_convergence replaces result.cost.

Fixes #910

Type of change

Please add a line in the relevant section of CHANGELOG.md to document the change (include PR #).

Important checks:

Please confirm the following before marking the PR as ready for review:

• No style issues: $ pre-commit run or $ nox -s pre-commit (see CONTRIBUTING.md for how to set this up to run automatically when committing locally, in just two lines of code)
• All tests pass: nox -s tests
• The documentation builds: nox -s doctest
• Code is commented for hard-to-understand areas
• Tests added that prove fix is effective or that feature works

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.86%. Comparing base (4628a65) to head (ef3bdf7).
⚠️ Report is 41 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #911      +/-   ##
===========================================
+ Coverage    89.41%   92.86%   +3.44%     
===========================================
  Files           63       73      +10     
  Lines         4857     5678     +821     
===========================================
+ Hits          4343     5273     +930     
+ Misses         514      405     -109     

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