Add more comments.

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

@@ -3,22 +3,44 @@ package org.nwapw.abacus.tree;
 import org.nwapw.abacus.number.NaiveNumber;
 import org.nwapw.abacus.number.NumberInterface;
 
+/**
+ * A node implementation that represents a single number.
+ */
 public class NumberNode extends TreeNode {
 
+    /**
+     * The number that is represented by this number node.
+     */
     private NumberInterface number;
 
+    /**
+     * Creates a number node with no number.
+     */
     public NumberNode(){
         number = null;
     }
 
+    /**
+     * Creates a new number node with the given double value.
+     * @param value the value to use.
+     */
     public NumberNode(double value){
         number = new NaiveNumber(value);
     }
 
+    /**
+     * Creates a new number node with the given string value, converted
+     * to a double.
+     * @param value the value.
+     */
     public NumberNode(String value){
         this(Double.parseDouble(value));
     }
 
+    /**
+     * Gets the number value of this node.
+     * @return the number value of this node.
+     */
     public NumberInterface getNumber() {
         return number;
     }