python constraint for input specification

[skhuang]

1. using eval of python to evaluate the single constraint, instead of lexical analysis and evaluate the expression in our own by
   new_val = random(); relace all $xx variables with the new_val in the A_str and call eval(A_str)
2. test if it works in former implementation
3. test for more complicated constraints: $1%2==0 (even number)