Fix "Complexity" issue in kyu_2/evaluate_mathematical_expression/evaluate.py

[ikostan]

Cyclomatic complexity is too high in function check_conditions. (11)

https://codeclimate.com/github/iKostanOrg/codewars/kyu_2/evaluate_mathematical_expression/evaluate.py#issue_6721b14b6578ac0001000941

[ikostan]

Cyclomatic Complexity

Cyclomatic Complexity corresponds to the number of decisions a block of code contains plus 1. This number (also called McCabe number) is equal to the number of linearly independent paths through the code. This number can be used as a guide when testing conditional logic in blocks.

Radon analyzes the AST tree of a Python program to compute Cyclomatic Complexity. Statements have the following effects on Cyclomatic Complexity:

Construct Effect on CC Reasoning

if +1 An if statement is a single decision.
elif +1 The elif statement adds another decision.
else +0 The else statement does not cause a new decision. The decision is at the if.
for +1 There is a decision at the start of the loop.
while +1 There is a decision at the while statement.
except +1 Each except branch adds a new conditional path of execution.
finally +0 The finally block is unconditionally executed.
with +1 The with statement roughly corresponds to a try/except block (see PEP 343 for details).
assert +1 The assert statement internally roughly equals a conditional statement.
Comprehension +1 A list/set/dict comprehension of generator expression is equivalent to a for loop.
Boolean Operator +1 Every boolean operator (and, or) adds a decision point.

Source: http://radon.readthedocs.org/en/latest/intro.html