Make the Reverse Polish Notation calculator work.

  We went through the implementation of an RPN calculator in class.
  Make it into a complete C++ program and make sure it works properly.
  And make it work for doubles, rather than just ints.

  Here's a small sample of the sort of thing that should work:
  (The dots at the end of each RPN expression are to mark the end of the expression)

  1 2 3 4 5 + + + + .
  result is 15

  12.3 45.6 + .
  result is 57.9

  1 2 + 3 + 4 + 5 + 10 / .
  result is 1.5

  1 2 * 3 4 * + 5 + .
  result is 19

  1 2 + 3 4 + * 5 * . 
  result is 105

Your program should be crash-proof. That means that it should continue running and
only give correct results no matter what bad inputs the user provides.