Replace usage of `connected_nodes` during the refactoring

[antonykamp]

During the refactoring See simulate-digital-rail/yaramo#40 we have to replace writing access of connected_nodes with connected_edges and edit calls of set_connection_left, set_connection_right and set_connection_head to take edges instead of nodes.

Acceptance criterion

• Replace writing access of connected_nodes with connected_edges
• Give set_connection_left the edge instead of a node
• Give set_connection_right the edge instead of a node
• Give set_connection_head the edge instead of a node