fix memory leak, remove unused code. (#274)

this.nerons[id] is a function, so ```this.neurons[id] = neuron```
will save neurons to this functions prototype and leak memory

#164

Author: Carl Hopf

src/Layer.js

@@ -197,7 +197,7 @@ export default class Layer {
 
 // adds a neuron to the layer
   add(neuron) {
-    this.neurons[neuron.ID] = neuron || new Neuron();
+    neuron = neuron || new Neuron();
     this.list.push(neuron);
     this.size++;
   }