Fix #Issue

Changes proposed in this pull request:

• Introduce new parameter MaximumAddCap (where MaximumAddCap=MaximumCap-InstalledCap) in C0.process_maximum_cap_constraint
• Deactivate optimization when MaximumCap<=InstalledCap in C0.process_maximum_cap_constraint
• Fix MaximumCap definition in RTD and include definition for MaximumAddCap
• Replace usage of MaximumCap in D1 with MaximumAddCap calculated in C0

The following steps were realized, as well (if applies):

• Use in-line comments to explain your code
• ❌ Write docstrings to your code (example docstring)
• For new functionalities: Explain in readthedocs
• ❌ Write test(s) for your new patch of code (pytests, assertion debug messages)
• Update the CHANGELOG.md
• Apply black (black . --exclude docs/)
• Check if benchmark tests pass locally (EXECUTE_TESTS_ON=master pytest)

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