Add formulas for bispectrum in fourier.py

[matteobachetti]

Not for the 1.0 release.
I had drafted these functions to work on my time series analysis classes, and Issue #639 was a good excuse to start thinking about using them in Stingray :)

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This change is 

[pep8speaks]

Hello @matteobachetti! Thanks for updating this PR. We checked the lines you've touched for PEP 8 issues, and found:

• In the file stingray/bispectrum.py:

Line 13:101: E501 line too long (102 > 100 characters)

Comment last updated at 2022-02-24 10:45:49 UTC

[codecov]

Codecov Report

❌ Patch coverage is 12.19512% with 36 lines in your changes missing coverage. Please review.
✅ Project coverage is 96.75%. Comparing base (8b4b3e7) to head (9bad0b5).
⚠️ Report is 1598 commits behind head on main.

Files with missing lines	Patch %	Lines
stingray/fourier.py	3.84%	25 Missing ⚠️
stingray/bispectrum.py	26.66%	11 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #640      +/-   ##
==========================================
- Coverage   97.21%   96.75%   -0.47%     
==========================================
  Files          41       41              
  Lines        7535     7576      +41     
==========================================
+ Hits         7325     7330       +5     
- Misses        210      246      +36     

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