Evaluation of expressions with variables

• interp.g4
• main.py
• Test files: p0 (pure expressions) and p1 (simple expressions with variables). Output:

          $ python main.py p0
          Result: 6.5
          Result: 1346.8125
          $ python main.py p1
          Result: 5.0
          Result: 42.0
          Result: 9.5
  

Note the use of @header to initialze a "global" variable; could also use this for imports and defining new functions.