Input line

Input lines are nobs which have no natural counterpart in the human neural system. They are only tools for the construction of a model. When plugged into an output line, they turn into a knot. Commands:
 * Input lines cannot have incoming pipes.
 * Input lines have always exactly one outgoing pipe.
 * Extend - the input line is extended with a new incoming dendrite with an input line at the tip. The original input line is transformed into a knot.
 * Select - Select all connected nobs
 * There is no explicit 'Delete' command for input lines. Deleting an input line would leave the outgoing pipe with an open end, which is not allowed. Instead you can delete the outpoing pipe. The input line at the start point of the pipe will disappear automatically.