take actual grid supply for cost calculation of fixed load

[jakobgemassmer]

Changes proposed in this pull request:

• only take actual grid supply for cost calculation of fixed load instead of original fixed load

The following steps were realized (required):

• [x] Correct linting with flake8 path_to_script
• [x] Check if tests pass locally with python -m pytest tests/

Also the following steps were realized (if applies):

• [ ] Write docstrings to your code
• [ ] Explain new functionalities in readthedocs
• [ ] Write test(s) for new patch of code
• [x] Check building of documentation with doc/make html