use generations instead of evaluations in optimisation plot

[AurelienJaquier]

also change colors of graph

makes it easier to read, I think that generation number is more meaningful than number of evaluations (the number of evaluations being the number of generations multiplied by the number of offsprings)

[codecov-commenter]

Codecov Report

Patch coverage: 13.33% and project coverage change: -0.06% ⚠️

Comparison is base (de30368) 61.47% compared to head (a94f976) 61.41%.
Report is 7 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #44      +/-   ##
==========================================
- Coverage   61.47%   61.41%   -0.06%     
==========================================
  Files          93       93              
  Lines        6185     6191       +6     
==========================================
  Hits         3802     3802              
- Misses       2383     2389       +6     

Files Changed	Coverage Δ
bluepyemodel/emodel_pipeline/plotting.py	15.70% <0.00%> (ø)
bluepyemodel/export_emodel/export_emodel.py	14.81% <18.18%> (-0.88%)	⬇️

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