Merge tree construction feature into master.

src/org/nwapw/abacus/tree/NumberNode.java

@@ -0,0 +1,25 @@
+package org.nwapw.abacus.tree;
+
+import org.nwapw.abacus.number.NaiveNumber;
+import org.nwapw.abacus.number.NumberInterface;
+
+public class NumberNode extends TreeNode {
+
+    private NumberInterface number;
+
+    public NumberNode(){
+        number = null;
+    }
+
+    public NumberNode(double value){
+        number = new NaiveNumber(value);
+    }
+
+    public NumberNode(String value){
+        this(Double.parseDouble(value));
+    }
+
+    public NumberInterface getNumber() {
+        return number;
+    }
+}