Evaluable update
  Evaluable interface:
    Changed the evaluate method to getValue;
    Added new evaluate method which returns an Evaluable;
    Updated Expression, Constant & Operator classes to fit the change.
  Constant class:
    Modified EConstant and PIConstant slightly to minimize redundancy.
  Operator class:
  	Added Sin and Asin operators;
  	First implementation of the operator invert method.