input: Write a python function to find the minimum value in a given BST.
  assert minValue(Node(10)) == 10
  assert minValue(Node(10, Node(5), Node(15))) == 5
  assert minValue(Node(20, Node(10, Node(5)), Node(30))) == 5
output: def minValue(root):
      current = root
      while current.left is not None:
          current = current.left
      return current.data