Reverse Polish Notation Calculator
A simple RPN Calculator with an intractable stack, undo, and basic calculation buttons.
Features: 
- Scrollable stack
- Drag and drop items in the stack
- Swipe to delete items from the stack
- Swap and Copy elements in the stack
- Undo 
- Conversion for Radians and Degrees
- Perform common and basic calculations
Tips:
- Pressing Enter while the input is empty will duplicate the value in row 1
- Long  press Undo to delete the Stack, Undo History, and Memory
- Swap/Copy 
     - Tap a value in the stack to select it.
     - Select a second value to swap with or select input when it is empty to copy it.
     - The first selection may also be copied to memory by pressing M-in after the first 
       selection is made.