Refactor is_linear_expression function of expr.py

[alisa-kirkinskaia]

1. Fix #121:

Fixed a bug in is_linear_expression related to expand() being very slow for some expressions. The bug is caused by the following line:

if not( (a-b).expand() == 0 or a.expand() == b.expand()):

In the fix we avoid comparing the values of a.expand() and b.expand() if (a-b).expand() is 0, as it already means the expression is linear.

2. Refactor is_linear_expression for better readability:

• Remove unused argument integral
• Rename variables
• Add docstring