Test and fix multiple inflows/outflows for energyConversion assets

Adresses #802, #532, #799, #599, #541, #372

Changes proposed in this pull request:

[ ] Update data of tests/benchmark_test_inputs/Feature_output_flows_as_list
[x] Update data of tests/benchmark_test_inputs/Feature_output_flows_as_list
[~] Add pytest for multiple inputs and outputs for C0.add_asset_to_asset_dict_for_each_flow_direction
[x] Add validity checks in C1 for multiple input- and output flows including tests
[x] Implement multiple input or output flows for D1.test_transformer_constant_efficiency_fix including tests
[ ] Implement multiple input or output flows for D1.test_transformer_constant_efficiency_optimize including tests
[ ] Fix issue regarding dispatch price in D1
[ ] Write benchmark test based on tests/benchmark_test_inputs/Feature_input_flows_as_list
[ ] Write benchmark test based on tests/benchmark_test_inputs/Feature_output_flows_as_list
[~] Update RTD explaining how multiple flows can be used

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)

Please mark above checkboxes as following:

:x: Check not applicable to this PR

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rl-institut / multi-vector-simulator