Fix variance() in summarise_draws()

[mjskay]

Summary

This PR is mainly to fix #219 by adding a variance.draws_array() and variance.draws_matrix() implementation so that variance() works properly with summarise_draws(). For example this works correctly (and should continue to do so on future versions of {distributional}):

> summarise_draws(example_draws(), variance)
# A tibble: 10 x 2
   variable variance
   <chr>       <dbl>
 1 mu           11.6
 2 tau          12.8
 3 theta[1]     39.7
 4 theta[2]     21.5
 5 theta[3]     46.2
 6 theta[4]     24.2
 7 theta[5]     25.9
 8 theta[6]     26.6
 9 theta[7]     27.7
10 theta[8]     27.6

There are two other minor fixes I made while I was in here:

• It adds support for casting from rvar to character vectors via vec_cast(x, character()) or as.character(x). This is mainly so that functions like paste() work as expected when passed rvars; e.g. this now works:

> x = rvar(rnorm(1000))
> paste("x =", x)
[1] "x = 0.01 ± 1"

• It adds skip_if_not_installed("caret") to all tests of rstar() since that function requires {caret}, but {caret} is only in "Suggests".

Since these are all minor changes, once tests / coverage passes if I don't hear any objections in the next week or so I'll merge.

Copyright and Licensing

By submitting this pull request, the copyright holder is agreeing to
license the submitted work under the following licenses:

• Code: BSD 3-clause (https://opensource.org/licenses/BSD-3-Clause)
• Documentation: CC-BY 4.0 (https://creativecommons.org/licenses/by/4.0/)

[codecov-commenter]

Codecov Report

Merging #221 (46fa6df) into master (87a823b) will increase coverage by 0.00%.
The diff coverage is 100.00%.

❗ Current head 46fa6df differs from pull request most recent head 434df88. Consider uploading reports for the commit 434df88 to get more accurate results

@@           Coverage Diff           @@
##           master     #221   +/-   ##
=======================================
  Coverage   94.32%   94.33%           
=======================================
  Files          41       41           
  Lines        2872     2875    +3     
=======================================
+ Hits         2709     2712    +3     
  Misses        163      163           

Impacted Files	Coverage Δ
R/as_draws_array.R	95.00% <100.00%> (+0.05%)	⬆️
R/as_draws_matrix.R	97.67% <100.00%> (+0.02%)	⬆️
R/rvar-cast.R	91.76% <100.00%> (+0.09%)	⬆️

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[mitchelloharawild]

{distributional} revdep checks have finished, and it seems that this is the only change to worse. Once this PR makes it to CRAN, everything should be good to go for a {distributional} update. 👍 🤞

[paul-buerkner]

I am happy with the PR from my side. After you have merged this, I will prepare a new release of posterior to CRAN.

[mjskay]

Sounds good, thanks all! I fixed a minor test case failure on R < 4 and am merging now (the failure of macOS devel on github ci is spurious).

[paul-buerkner]

Today I submitted the new version of posterior to CRAN and it should be released in a few days.