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):

• [ ] 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)
• [x] Update the CHANGELOG.md
• [x] Apply black (black . --exclude docs/)
• [x] Check if benchmark tests pass locally (EXECUTE_TESTS_ON=master pytest)

Please mark above checkboxes as following:

• [ ] Open
• [x] Done

:x: Check not applicable to this PR

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