Individual contributions to the freq-freq cross

[dpole]

It applies only to CorrelatedFactorizedCrossSpectrum

[codecov-io]

Codecov Report

Merging #31 into master will decrease coverage by 0.31%.
The diff coverage is 33.33%.

@@            Coverage Diff             @@
##           master      #31      +/-   ##
==========================================
- Coverage   64.01%   63.69%   -0.32%     
==========================================
  Files           4        4              
  Lines         289      292       +3     
==========================================
+ Hits          185      186       +1     
- Misses        104      106       +2

Impacted Files	Coverage Δ
fgspectra/cross.py	53.94% <33.33%> (-0.85%)	⬇️

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[dpole]

@xzackli and @mabitbol , I don't even remember who requested this feature, but I guess we can either merge or delete this PR