fix npe

Author: Lukas Molzberger

src/main/java/network/aika/Document.java

@@ -24,7 +24,6 @@
 import network.aika.neuron.INeuron;
 import network.aika.neuron.Synapse;
 import network.aika.neuron.activation.Activation;
-import network.aika.neuron.activation.Activation.Link;
 import network.aika.neuron.activation.Activation.Option;
 import network.aika.neuron.activation.Candidate;
 import network.aika.neuron.activation.Position;

src/main/java/network/aika/neuron/INeuron.java

@@ -551,6 +551,12 @@ public void propagate(Activation act) {
 
 
     public int compareTo(INeuron n) {
+        if (this == n) return 0;
+        if (this == MIN_NEURON) return -1;
+        if (n == MIN_NEURON) return 1;
+        if (this == MAX_NEURON) return 1;
+        if (n == MAX_NEURON) return -1;
+
         if (provider.id < n.provider.id) return -1;
         else if (provider.id > n.provider.id) return 1;
         else return 0;