Connect valves to branch elements

[dlohmeier]

This discussion was created from issue #117:

Similar to pandapower it could be possible to create different valve types that allow to connect valves between a junction and a branch element (e.g. a pipe). It would require a few changes in the valve component:

• add a column like "et" in pandapower switch table
• add internal nodes for node-to-branch valves in create_node_lookups of valve component
• consider these internal nodes in all methods of the valve component