Prints the Decision Tree in a pretty way.

def print_tree(root, space=' '):
    """Prints the Decision Tree in a pretty way.
    """

    if isinstance(root, Leaf):
        print(space + "Prediction: " + str(root.most_frequent))
        return

    print(space + str(root.question))

    print(space + "--> True:")
    print_tree(root.true_branch, space+'  ')

    print(space + "--> False:")
    print_tree(root.false_branch, space+'  ')