Update SAT object

  - Update SAT class with easy to read literal and variable operations.
Update Solver object

  - Add isClauseConsistent() to the Solver class to check if a given clause is consistent (evaluates to True) for the assignment.
  - Update isSolved() in Solver class so it checks if all clauses are consistent.
  - Update Solver class so the SAT object is accessible by looking at the 'SAT' property (instead of 'sat').