Visu sorting problem

[erikadudki]

• now Xvalues (conditions) are sorted monotonically increasing

closes #393, closes #330,
(@MerktSimon does this also close #207?)

[codecov]

Codecov Report

Merging #396 into develop will increase coverage by 0.02%.
The diff coverage is 100%.

@@             Coverage Diff             @@
##           develop     #396      +/-   ##
===========================================
+ Coverage    77.77%   77.79%   +0.02%     
===========================================
  Files           22       22              
  Lines         2070     2072       +2     
  Branches       476      476              
===========================================
+ Hits          1610     1612       +2     
  Misses         340      340              
  Partials       120      120

Impacted Files	Coverage Δ
petab/visualize/helper_functions.py	88.73% <100%> (+0.05%)	⬆️
petab/visualize/plotting_config.py	48.19% <100%> (+0.63%)	⬆️

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[MerktSimon]

Looks good, just remove the two not needed lines