Fix/unused storage capacity flow

[smartie2076]

Fix #847

Changes proposed in this pull request: Planned changes:

• [x] Change AE-grid-battery benchmark test to read from json files
• [x] Change AE-grid-battery benchmark test for one case where storage can not be used and one where it can be used, so that this is made explicit
• [x] Adapt AE-grid-battery so that the issue described in #847 leads to a failing pytest
• [x] Determine if STORAGE_CAPACITY/FLOW or TOTAL_FLOW is used in E, if not, add exception that TOTAL_FLOW etc. for STORAGE_CAPACITY is NaN. Make sure that SOC can still be calculated.
• [x] Check PR #791 and determine why method not applied to flows of the storage
• [x] Apply precision limit to storage flows

Extra changes:

• [x] Add check if storage_filename is valid

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

• [x] Use in-line comments to explain your code
• [x] Write docstrings to your code (example docstring)
• [x] For new functionalities: Explain in readthedocs
• [x] 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)

[smartie2076]

There is a double structure to determine the dispatch of assets - via E1.get_timeseries_per_bus with E1.get_flow() assertions, and thus E1.cut_below_micro is applied twice for each flow. This should rather be merged into the other functions.

[smartie2076]

@ciaradunks the PR is ready for review now! :)

[smartie2076]

@smartie2076 I've had a look through each commit and it all looks fine to me!

Oh, nice, no changes required =D