Fix #773

Changes proposed in this pull request:

• Added net zero energy KPI degree of NZE with E3.equation_degree_of_net_zero_energy() and E3.add_degree_of_net_zero_energy()
• Added tests for E3.equation_degree_of_net_zero_energy() and E3.add_degree_of_net_zero_energy()
• Added information about degree of NZE to RTD

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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• Done

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