Fix #550

Changes proposed in this pull request:

• Now diff can also be of type int

Notes:

• There are still some pytests missing for the function, specifically for prices/tariffs provided as a timeseries

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

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