fixes calculation of marginal cost and output_before

[maurerle]

• add tests for base_unit
• remove global start and end in flex storage test

We still had an issue in the marginal_cost calculation which I fixed in a wrong way in #245
This is now tested and fixed.
While testing, I found that the product_type was not used in the get_output_before function too.

[codecov]

Codecov Report

Attention: 2 lines in your changes are missing coverage. Please review.

Comparison is base (c32a623) 78.21% compared to head (cc74f6d) 78.33%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #250      +/-   ##
==========================================
+ Coverage   78.21%   78.33%   +0.11%     
==========================================
  Files          39       39              
  Lines        4214     4218       +4     
==========================================
+ Hits         3296     3304       +8     
+ Misses        918      914       -4     

Flag	Coverage Δ
pytest	78.33% <90.47%> (+0.11%)	⬆️

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Files	Coverage Δ
assume/common/units_operator.py	90.95% <100.00%> (+0.40%)	⬆️
assume/markets/base_market.py	87.56% <100.00%> (+0.06%)	⬆️
assume/common/base.py	80.26% <85.71%> (+1.33%)	⬆️

... and 1 file with indirect coverage changes

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