add objective module and objective class in order to allow maximization problems

[FabianHofmann]

closes #168

[codecov]

Codecov Report

Attention: 7 lines in your changes are missing coverage. Please review.

Comparison is base (74ce2d2) 87.30% compared to head (ed67bb4) 87.59%.
Report is 16 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #170      +/-   ##
==========================================
+ Coverage   87.30%   87.59%   +0.29%     
==========================================
  Files          14       15       +1     
  Lines        3119     3225     +106     
  Branches      707      742      +35     
==========================================
+ Hits         2723     2825     +102     
- Misses        289      290       +1     
- Partials      107      110       +3     

Files	Coverage Δ
linopy/__init__.py	100.00% <100.00%> (ø)
linopy/expressions.py	89.39% <100.00%> (+0.38%)	⬆️
linopy/io.py	78.93% <100.00%> (+0.30%)	⬆️
linopy/matrices.py	100.00% <100.00%> (ø)
linopy/solvers.py	89.93% <100.00%> (-0.22%)	⬇️
linopy/model.py	88.51% <95.83%> (+0.21%)	⬆️
linopy/objective.py	93.68% <93.68%> (ø)

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