Change sliding window width parameter to tau=3

[rccreswell]

For the Rt example notebook, use a shorter sliding window to avoid introducing significant discrepancy (at certain time periods) between Poisson and Negative Binomial results which is occurring at the current choice of sliding window width.

I changed the parameter from tau=6 to tau=3.

Closes #1735

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.54%. Comparing base (1af369a) to head (4f2e2e5).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1736   +/-   ##
=======================================
  Coverage   99.54%   99.54%           
=======================================
  Files          97       97           
  Lines        9762     9762           
=======================================
  Hits         9718     9718           
  Misses         44       44           

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