.cumsum for Variables and LinearExpression

[hblunck]

Addresses #145

Implements (to the best of my knowledge) .cumsum methods for Variable and LinearExpression objects. No implementation is provided yet for e.g. LinearExpressionGroupBy objects.

[codecov]

Codecov Report

Patch coverage: 69.23% and project coverage change: -0.09 ⚠️

Comparison is base (e9c9efc) 87.03% compared to head (3a56e6e) 86.95%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #147      +/-   ##
==========================================
- Coverage   87.03%   86.95%   -0.09%     
==========================================
  Files          14       14              
  Lines        3101     3112      +11     
  Branches      707      710       +3     
==========================================
+ Hits         2699     2706       +7     
- Misses        294      296       +2     
- Partials      108      110       +2     

Impacted Files	Coverage Δ
linopy/expressions.py	87.45% <66.66%> (-0.33%)	⬇️
linopy/variables.py	90.75% <75.00%> (-0.18%)	⬇️

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[FabianHofmann]

Very nice, thanks @hblunck. Just one small thing

[FabianHofmann]

and would you mind adding a small test to test_linear_expression? If you are time-constrained, I can also take over.

[hblunck]

Hey @FabianHofmann , I added some tests, but they fail. On inspection (i.e. printing), the linear expressions are exactly what I would expect and they are also equal. But assert_linequal is throwing errors at me that I cannot decipher. I think I would have to hand it over to you at this point.

[FabianHofmann]

I don't have a good solution. Let's keep it simplistic then