Feature/scenarios results

[FBumann]

No description provided.

[FBumann]

Improvements

• Add scenarios to CalculationResults
• Add Scenarios to all plotting options, defaulting to the first scenario available
• Add the scenario to the plot title and filename if scenario is used
• Improve filter_solution: Filter by time steps, scenarios and variable dims
• Add options mode to node_balance() and corresponding plotting methods, to allow to get/plot the flow hours instead of the flow_rate. The plot_pie() always does that
• Improve docstrings in general