Add exponentially weighted moving average

[mengelbart]

We use this to track the EWMA of the delivery rate and to estimate when we are "close" to convergence, which is defined as being within three standard deviations of the rate when we were in the decrease state.
https://datatracker.ietf.org/doc/html/draft-ietf-rmcat-gcc-02#section-5.5

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (1d19464) to head (71ab935).
⚠️ Report is 3 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff            @@
##            master       #29   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            1         2    +1     
  Lines           31        49   +18     
=========================================
+ Hits            31        49   +18     

Flag	Coverage Δ
go	100.00% <100.00%> (ø)

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[JoTurk]

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