New autoregressive rules

[bvdmitri]

This PR reimplements some rules for Autoregressive node to be more efficient both in terms of execution times and memory allocation. In addition, this PR adds an example of Autoregressive model in the documentation.

[codecov-commenter]

Codecov Report

Merging #55 (128b60a) into master (de7aff4) will decrease coverage by 0.39%.
The diff coverage is 20.37%.

@@            Coverage Diff             @@
##           master      #55      +/-   ##
==========================================
- Coverage   50.86%   50.47%   -0.40%     
==========================================
  Files         126      126              
  Lines        3761     3802      +41     
==========================================
+ Hits         1913     1919       +6     
- Misses       1848     1883      +35     

Impacted Files	Coverage Δ
src/algebra/helpers.jl	8.33% <ø> (-91.67%)	⬇️
src/rules/autoregressive/marginals.jl	0.00% <0.00%> (ø)
src/nodes/autoregressive.jl	16.00% <22.22%> (+16.00%)	⬆️
src/distributions/mv_normal_mean_covariance.jl	97.91% <100.00%> (ø)
src/distributions/mv_normal_mean_precision.jl	97.61% <100.00%> (-0.06%)	⬇️
...distributions/mv_normal_weighted_mean_precision.jl	97.77% <100.00%> (ø)

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