Refactors

- getNode method to handle invalid node index and improve error handling
- update parameter description in SimulationMultiple constructor
- enhance calcError method in StatisticsSummary for better clarity and functionality.

src/main/java/net/berack/upo/valpre/sim/Net.java

@@ -127,7 +127,8 @@ public final class Net implements Iterable<ServerNode> {
      * @return the node
      */
     public ServerNode getNode(String name) {
-        return this.servers.get(this.getNodeIndex(name));
+        var index = this.getNodeIndex(name);
+        return index < 0 ? null : this.servers.get(index);
     }
 
     /**