Enable use of priors with least squares optimizers

[FFroehlich]

Adds res + sres functions to priors where possible. Extends prior testcases to include multi-dimensional problems.

[codecov-commenter]

Codecov Report

Merging #745 (03e5ec3) into develop (c62ebcd) will increase coverage by 0.43%.
The diff coverage is 89.47%.

@@             Coverage Diff             @@
##           develop     #745      +/-   ##
===========================================
+ Coverage    89.47%   89.90%   +0.43%     
===========================================
  Files           99       99              
  Lines         6734     6786      +52     
===========================================
+ Hits          6025     6101      +76     
+ Misses         709      685      -24     

Impacted Files	Coverage Δ
pypesto/startpoint/assign.py	92.30% <ø> (+7.69%)	⬆️
pypesto/objective/base.py	87.12% <75.00%> (+5.03%)	⬆️
pypesto/objective/priors.py	86.95% <90.27%> (+6.50%)	⬆️
pypesto/optimize/optimizer.py	90.38% <0.00%> (+0.22%)	⬆️
pypesto/petab/importer.py	89.56% <0.00%> (+4.39%)	⬆️

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[yannikschaelte]

looks very good.