Allow specifying file format for visualization

[dweindl]

So far, it was only possible to generate PNGs. This is still the default, but can be changed to any other format supported by matplotlib

[codecov-commenter]

Codecov Report

Merging #85 (48dc5c8) into develop (918e1a4) will decrease coverage by 0.01%.
The diff coverage is 100.00%.

@@             Coverage Diff             @@
##           develop      #85      +/-   ##
===========================================
- Coverage    79.59%   79.57%   -0.02%     
===========================================
  Files           26       26              
  Lines         2940     2938       -2     
  Branches       695      695              
===========================================
- Hits          2340     2338       -2     
  Misses         423      423              
  Partials       177      177              

Impacted Files	Coverage Δ
petab/visualize/plot_data_and_simulation.py	83.33% <100.00%> (-0.33%)	⬇️
petab/visualize/plotter.py	78.03% <100.00%> (ø)

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