created smart constructors (var,not,and,or), changed variable from string to int (variable number)
changed variable from string to int (variable number)
removed old test cases
added id (int) to OBDDs, constants for terminals
added function assign, which evaluates a Formula
changed hashmap for equivalence (Integer instead of OBDD), deleted unused getters and setter)
added method to get a Formulas variables
added some test cases for Formula evaluation