Input line

Input neurons 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 neuron, they turn into regular neurons. Commands:
 * Input neurons cannot have incoming pipes.
 * Input neurons can have only one outgoing pipe, just like ordinary neurons.
 * Input neurons without an outgoing pipe can exist in a model, but this makes sense only as a temporary state.
 * New input neuron - create a new input neuron without any connections
 * Delete - delete the input neuron
 * Disconnect - break all connections of the input neuron
 * Add outgoing dendrite - only available if none exists already
 * Pulse rate - provide frequency for a regular pulse
 * Select nob/Deselect nob - add to/remove from selection