Fix/plots labels

[Bachibouzouk]

Fix #557

Changes proposed in this pull request:

• Rename the function in F1 module plot_flows to plot_instant_power
• Change flow to power in the instanteous power figures
• Labels on capacity barplot bars

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

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

_{For more information on how to contribute check the CONTRIBUTING.md.}

[Bachibouzouk]

@ursulaelmir - you can approve and merge then :)