added sorting to appending. closes #378

[PaulJonasJost]

closes #378

sorting when reading from file (at least for hdf5) is already enforced.

[codecov-commenter]

Codecov Report

Merging #668 (9f9826c) into develop (4fd93af) will increase coverage by 44.55%.
The diff coverage is 100.00%.

@@             Coverage Diff              @@
##           develop     #668       +/-   ##
============================================
+ Coverage    42.98%   87.54%   +44.55%     
============================================
  Files           95       95               
  Lines         6134     6135        +1     
============================================
+ Hits          2637     5371     +2734     
+ Misses        3497      764     -2733     

Impacted Files	Coverage Δ
pypesto/result.py	88.67% <100.00%> (+29.06%)	⬆️
pypesto/predict/result.py	33.33% <0.00%> (-61.12%)	⬇️
pypesto/ensemble/utils.py	21.21% <0.00%> (-36.37%)	⬇️
pypesto/petab/importer.py	70.25% <0.00%> (-15.19%)	⬇️
pypesto/predict/amici_predictor.py	82.71% <0.00%> (-14.82%)	⬇️
pypesto/objective/amici_util.py	75.43% <0.00%> (-7.02%)	⬇️
pypesto/petab/pysb_importer.py	73.33% <0.00%> (-6.67%)	⬇️
pypesto/objective/amici_calculator.py	90.00% <0.00%> (ø)
pypesto/objective/base.py	87.71% <0.00%> (+2.33%)	⬆️
pypesto/startpoint/util.py	87.50% <0.00%> (+2.50%)	⬆️
... and 62 more

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