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 :)

[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

Merging #640 (9bad0b5) into main (8b4b3e7) will decrease coverage by 0.46%.
The diff coverage is 12.19%.

@@            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     

Impacted Files	Coverage Δ
stingray/fourier.py	93.28% <3.84%> (-6.45%)	⬇️
stingray/bispectrum.py	87.95% <26.66%> (-6.09%)	⬇️

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