Fix #523
Fix #768

We had issues with the SOC being out of bounds with [0,1] and marginal negative flows. This PR fixes both issues. The marginal negative flows are rounded to 0 (threshold: -10^-6), large negative values raise a warning and are unchanged. Marginal positive flows (reason for SOC out of bounds) are also rounded to 0.

Compared to SOC in #768, you can see that in the now created simulation_report.pdf the SOC is always 0.

Changes proposed in this pull request:
[x] Add precision issue in RTD
[x] Function E1.cut_below_pico to round decision variables (capacities, flows) below threshold of plus/minus 10^-6 to 0, add warnings if negative value larger then threshold (invalid result). Includes pytests (#)
[x] Call E1.cut_below_pico in E1.get_flows, E1.get_optimal_cap and E1.get_storage_results (#)

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)