Fix #295

Changes proposed in this pull request:

• loss_rate of storages in D1 defined as 1-efficiency instead of as efficiency of the storage capacity (see storage_xx.csv files)
• efficiency of storage_capacity in storage_xx.csvnow actually is defined as the storages' efficiency/ability to hold charge over time
• Adapted efficiency of storage_capacity in all provided benchmark tests and inputs to 1-value
• Documented the change of efficiency of storage_capacity as actual efficiency/ability to hold charge over time in RTD
• Added a error message if the efficiency of storage capacity is 0 and logging.warning if the efficiency is < 0.2, to help users to spot major change when using old files

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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