Fix v2g discharge power in balanced_market

[mosc5]

Fixes #175

Changes proposed in this pull request:

• Discharge power for V2G in the balanced_market strategy now considers the charging station power

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/
• [ ] Add the description of your changes to the CHANGELOG.md in subsection with release name ## [Unreleased] and state the PR number with #

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
• [ ] Check building of documentation with doc/make html