Simplification of expression

[ayush9398]

This partially solves issue #141

• Added property reduced in the Expression class.
• Added a class method, get_reduced() to simplify the expression.
• If the expression is simplified to a single token, the class type changes from Expression to appropriate Function class.

[8hantanu]

Can you add some tests for the above? ~~Also, note you might need to add an extra check if the expression needs to be simplified. For example, when solving for y in x - 2yz + 1= 0 the resultant expression y=-0.5*((-1.0-x)/z) can be reduced but must not be reduced.~~

[ayush9398]

Hey @8hantanu , I have added the test function.