Add incoming dendrite

This command can be applied to The context menue of these nobs offers the command, if the before mentioned conditions are fulfilled.
 * Pipes: By adding an incoming dendrite to a pipe, a joining knot is created. Two incoming pipes join into one outgoing pipe.
 * Knots: As knots cannot have more than three connected pipes, only kink knots (one incoming, one outgoing pipe) accept another incoming dendrite.
 * Output neurons: Any number of incoming dendrites can go into an output neuron.

The command cannot be applied to input neurons because by definition they do not have incoming dendrites.