Implement std(::MultivariateDistribution)

[bicycle1885]

This implements std for multivariate distributions as an element-wise square root of variance: std(d) = sqrt.(var(d)).

[codecov-commenter]

Codecov Report

Merging #1352 (4b25a95) into master (a0cb096) will increase coverage by 0.84%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #1352      +/-   ##
==========================================
+ Coverage   82.23%   83.07%   +0.84%     
==========================================
  Files         116      117       +1     
  Lines        6635     6766     +131     
==========================================
+ Hits         5456     5621     +165     
+ Misses       1179     1145      -34     

Impacted Files	Coverage Δ
src/multivariate/dirichlet.jl	65.97% <100.00%> (+1.08%)	:arrow_up:
src/multivariates.jl	72.15% <100.00%> (+1.48%)	:arrow_up:
src/univariate/discrete/poisson.jl	74.07% <0.00%> (-3.35%)	:arrow_down:
src/multivariate/mvtdist.jl	60.41% <0.00%> (-1.59%)	:arrow_down:
src/univariate/discrete/categorical.jl	75.00% <0.00%> (-1.55%)	:arrow_down:
src/univariate/continuous/generalizedpareto.jl	78.20% <0.00%> (-1.55%)	:arrow_down:
src/univariate/discrete/bernoulli.jl	89.23% <0.00%> (-1.54%)	:arrow_down:
...rc/univariate/discrete/noncentralhypergeometric.jl	91.15% <0.00%> (-1.21%)	:arrow_down:
src/univariate/continuous/chi.jl	94.59% <0.00%> (-0.76%)	:arrow_down:
src/univariate/discrete/binomial.jl	92.94% <0.00%> (-0.47%)	:arrow_down:
... and 33 more

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[st--]

@bicycle1885 are you still interested in finishing this PR? @devmotion made a reasonable code suggestion that you would just need to accept, the other thing that seems missing is "add std to the documentation of multivariate distributions" - @devmotion could you clarify what you mean by that ?

[devmotion]

@bicycle1885 are you still interested in finishing this PR? @devmotion made a reasonable code suggestion that you would just need to accept, the other thing that seems missing is "add std to the documentation of multivariate distributions" - @devmotion could you clarify what you mean by that ?

I addressed both. I also added a fix for Dirichlet, let's see if tests pass.

[st--]

@devmotion tests are passing, should it get merged now ?

[devmotion]

I don't want to approve and merge my own changes. This does not feel right.

[azev77]

@devmotion @st-- in open source ppl write PRs then forget about them, moving on to other things ALL THE TIME! If we wait for people, we'll never get anything done