Added checkSyntax function to check if standard syntax as defined in user manual is followed

[vidhanarya]

Fixed syntax for input was not defined earlier as mentioned in issue #140, in this commit a function has been added which will check the given inputEquation if it follows the syntax defined in user manual. Example:

Before the function was added -

(aeroenv) vidhanarya@macbookpro:visma$ python main.py 
>>> simplify(2 + sin2)
INPUT: 2.0 + 2.0
OPERATION: simplify
OUTPUT: 2.0 + 2.0

2.0 + 2.0


>>> simplify(2 + sin())
INPUT: 2.0 + 
OPERATION: simplify
OUTPUT: 2.0 + 

2.0 + 


>>> simplify(2 + .4)
INPUT: 2.0 + 4.0
OPERATION: simplify
OUTPUT: 6.0

2.0 + 4.0

(Adding 4.0 and 2.0)
6.0


>>> simplify(.2 + 4)
INPUT: 2.0 + 4.0
OPERATION: simplify
OUTPUT: 6.0

2.0 + 4.0

(Adding 4.0 and 2.0)
6.0

After adding function -

>>> simplify(2 + sin2)
Invalid Expression
>>> simplify(2 + sin())
Invalid Expression
>>> simplify(2 + .4)
Invalid Expression
>>> simplify(.2 + 4)
Invalid Expression