Doc(Tree): Add solution to doctest in balance

mapytex/calculus/core/tree.py

@@ -702,6 +702,23 @@ class Tree(object):
         >>> t = Tree.from_str("1+2+3+4+5*6*7*8*9")
         >>> bal_t = t.balance()
         >>> print(bal_t)
+        +
+         > +
+         | > +
+         | | > +
+         | | | > 1
+         | | | > 2
+         | | > 3
+         | > 4
+         > *
+         | > *
+         | | > *
+         | | | > 5
+         | | | > 6
+         | | > 7
+         | > *
+         | | > 8
+         | | > 9
         """
         try:
             l_depth = self.left_value.depth()