Replace group with tags

[wlandau]

Summary

Before, we had

drake_plan(
  x = target(
    command,
    transform = map(y = c(1, 2)),
    group = my_group
  )
)

The whole separate "group" slot was confusing. It conflicted with the term "grouping variable", which describes y above, and it's a dangling slot of target().

This PR cleans up and completes the idea. Now, we have tags, which are grouping variables added post-hoc.

• In-tags: columns with the target names we start with.
• Out-tags: columns with the new target names that the transformation generates.

library(drake)
drake_plan(
  x = target(
    command,
    transform = map(y = c(1, 2), .tag_in = from, .tag_out = c(to, out))
  ),
  trace = TRUE
)
#> # A tibble: 2 x 7
#>   target command y     x     from  to    out  
#>   <chr>  <chr>   <chr> <chr> <chr> <chr> <chr>
#> 1 x_1    command 1     x_1   x     x_1   x_1  
#> 2 x_2    command 2     x_2   x     x_2   x_2

^{Created on 2019-01-20 by the reprex package (v0.2.1)}

Above,

• x and y are implicit out-tags.
• from is an explicit in-tag.
• to and out are explicit out-tags.

The whole concept is intuitive and symmetric. Users can chain tags together to create much more complicated dependency graphs from individual calls to drake_plan().

Related

#233

Checklist

• I have read drake's code of conduct, and I agree to follow its rules.
• I have listed any substantial changes in the development news.
• I have added testthat unit tests to tests/testthat to confirm that any new features or functionality work correctly.
• I have tested this pull request locally with devtools::check()
• This pull request is ready for review.
• I think this pull request is ready to merge.

[codecov-io]

Codecov Report

Merging #688 into master will not change coverage.
The diff coverage is 100%.

@@          Coverage Diff          @@
##           master   #688   +/-   ##
=====================================
  Coverage     100%   100%           
=====================================
  Files          72     72           
  Lines        5715   5723    +8     
=====================================
+ Hits         5715   5723    +8

Impacted Files	Coverage Δ
R/api-plan.R	100% <ø> (ø)	⬆️
R/api-dsl.R	100% <100%> (ø)	⬆️
R/utils-utils.R	100% <100%> (ø)	⬆️

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