Add min/max load transformer and reducable demand as assets

[Bachibouzouk]

Fix #Issue

Changes proposed in this pull request:

• see changelog

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

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• 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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