Evaluation of atoms.

Passes test 1 - 3.
Type checks.

Passes tests 4-5.
Added arithmetic + boolean operations.

Passes all test from tests/test_2_evaluating_simple_expressions.py . Also added exception cases for arothmetic with not only integers.
Fixed a bug.

There were no test cases for handling nested expressions, so I wrote one and discovered a bug. Now it handles nested arithmetic expression as well.