style: cleanup catalan_numbers.cpp

[vil02]

Description of Change

catalan_numbers.cpp has a quite strange design and a memory leak. This PR fixes that.

Checklist

• Added description of change
• Added tests and example, test must pass
• Added documentation so that the program is self-explanatory and educational - Doxygen guidelines
• Relevant documentation/comments is changed or added
• PR title follows semantic commit guidelines
• Search previous suggestions before making a new one, as yours may be a duplicate.
• I acknowledge that all my contributions will be made under the project's license.

Notes:
Cleanup catalan_numbers.cpp.