feat: add new fft algorithm

[ikocs]

Description of change A new direct and inverse fast Fourier transform algorithm has been added. The new algorithm uses templates. During development,

Checklist

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

Notes: I also discovered that the old algorithm did not work correctly, and the test data was entered incorrectly. An independent check can be carried out using an online calculator.

I suggest replacing the old files with new ones, but have not yet removed them from the repository.