Add autoplot() support for "groups" column role in rsmp("cv")

[pat-s]

fixes #157

library("mlr3verse")
#> Loading required package: mlr3
library("mlr3spatiotempcv")
library(data.table)
set.seed(42)

# create dataset with blocking from example dataset
task <- tsk("ecuador")
data_raw <- task$backend$data(1:task$nrow, task$feature_names)
group <- as.factor(sample(c("class1", "class2", "class3", "class4", "class5", "class6", "class7", "class8"),
  task$nrow,
  replace = TRUE
))
task$cbind(data.table("group" = group))
task$set_col_roles("group", roles = "group")

cv <- rsmp("cv", folds = 4)$instantiate(task)

# autoplot(cv, task)
autoplot(cv, task, fold_id = 1)

^{Created on 2021-09-12 by the reprex package (v2.0.1)}

[codecov-commenter]

Codecov Report

Merging #158 (fae5693) into main (133235c) will increase coverage by 0.06%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #158      +/-   ##
==========================================
+ Coverage   82.05%   82.11%   +0.06%     
==========================================
  Files          28       28              
  Lines        2312     2321       +9     
==========================================
+ Hits         1897     1906       +9     
  Misses        415      415              

Impacted Files	Coverage Δ
R/autoplot.R	53.27% <100.00%> (+0.63%)	⬆️

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