Add postfix expression evaluation

[nisaruj]

(Put an X inside the [ ] to denote check mark [X].)

• If creating a new file :

  • added links to it in the README files ?
  • included tests with it ?
  • added description (overview of algorithm, time and space compleixty, and possible edge case) in docstrings ?

• if done some changes :

  • wrote short description in the PR explaining what the changes do ?
  • Fixes #[issue number] if related to any issue

• other

[coveralls]

Pull Request Test Coverage Report for Build 760

• 0 of 20 (0.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.2%) to 71.282%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
algorithms/stack/evaluate_postfix.py	0	20	0.0%

Totals
Change from base Build 753:	-0.2%
Covered Lines:	4304
Relevant Lines:	6038

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💛 - Coveralls