Add stochastic triple smoothing backend

[gaurav-arya]

TODO:

• finish updating tests (including array indexing, propagate interface, filter state)
• add to benchmarks
• type stability
• similar_type does not give concrete type (needs Improve interface for conversion, promotion of backend types #83)

[codecov-commenter]

Codecov Report

Patch coverage: 96.19% and project coverage change: +0.84 🎉

Comparison is base (8a67258) 89.49% compared to head (1a85493) 90.33%.

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Additional details and impacted files

@@            Coverage Diff             @@
##             main      #81      +/-   ##
==========================================
+ Coverage   89.49%   90.33%   +0.84%     
==========================================
  Files          12       12              
  Lines         571      621      +50     
==========================================
+ Hits          511      561      +50     
  Misses         60       60              

Impacted Files	Coverage Δ
src/finite_infinitesimals.jl	100.00% <ø> (ø)
src/backends/smoothed.jl	86.48% <87.87%> (+4.66%)	⬆️
src/backends/dict.jl	92.06% <100.00%> (ø)
src/backends/pruned.jl	91.30% <100.00%> (ø)
src/backends/pruned_aggressive.jl	91.07% <100.00%> (ø)
src/discrete_randomness.jl	97.29% <100.00%> (+3.17%)	⬆️
src/general_rules.jl	93.85% <100.00%> (+1.07%)	⬆️
src/propagate.jl	85.00% <100.00%> (ø)
src/smoothing.jl	90.47% <100.00%> (-0.44%)	⬇️

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