Division and multiplication evaluating to 0.0 in unit tests

[mbartelsm]

In unit tests for eval_postfix, division and multiplication always evaluate to 0.0, even division by 0.
However, when running from main with the entire pipeline, it works as expected.

Ideas:

• Review unit tests
• Implement tests for other functions
• Compare, perhaps test implementation is wrong-

[mbartelsm]

Two changes were necessary:

1. Unit tests had a typo
2. Implementation of eval_postfix had an error that caused it to restore consumed items from the expression stack.

This issue is now fixed