Added functionality to check syntax of input equation

[vidhanarya]

Before generating tokens, tokenizer will check for the syntax of the input equation if it matches standards defined in User Manual -> Syntax Guidelines as described in issue #140 .

Example:

Before adding function-

(aeroenv) vidhanarya@macbookpro:visma$ python main.py 
>>> simplify(.2 + 4)
INPUT: 2.0 + 4.0
OPERATION: simplify
OUTPUT: 6.0

2.0 + 4.0

6.0
(Adding 4.0 and 2.0)


>>> simplify(2 + sin2)
INPUT: 2.0 + 2.0
OPERATION: simplify
OUTPUT: 2.0 + 2.0

2.0 + 2.0

When function was added-

(aeroenv) vidhanarya@macbookpro:visma$ python main.py 
>>> simplify(log2)
Invalid Expression
>>> addition(log2+log3)
Invalid Expression
>>> simplify(log(2))
INPUT: log(2.0)
OPERATION: simplify
OUTPUT: log(2.0)

log(2.0)


>>> simplify(0.2 + .4)
Invalid Expression
>>> simplify(.3 + 4)
Invalid Expression

[vidhanarya]

Closing this PR and opening a clean PR with 1 commit (Addition of checkSyntax function and test cases)