Fix calculation of MGA cost bound

[koen-vg]

Changes proposed in this Pull Request

I uncovered another issue with the MGA cost bound calculation; hopefully it really should be fixed now. The code would previously sometimes give the wrong number, but potentially only for a subset of components including already installed capacities or something. Anyway, the addition of the n.statistics.capex() and n.statistics.opex() dataframes doesn't work as-is and they need to be summed before adding the totals.

Checklist

• Code changes are sufficiently documented; i.e. new functions contain docstrings and further explanations may be given in doc.
• Unit tests for new features were added (if applicable).
• Newly introduced dependencies are added to environment.yaml, environment_docs.yaml and setup.py (if applicable).
• A note for the release notes doc/release_notes.rst of the upcoming release is included.
• I consent to the release of this PR's code under the MIT license.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 66.24%. Comparing base (baa242b) to head (d3d7870).

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #847   +/-   ##
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  Coverage   66.24%   66.24%           
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  Files          26       26           
  Lines        7063     7063           
  Branches     1418     1418           
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  Hits         4679     4679           
  Misses       2102     2102           
  Partials      282      282           

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[FabianHofmann]

thanks @koen-vg for the very digestible PRs