`.stat_over_list()` fails for arrays with more than two elements

[andreassoteriadesmoj]

Function .stat_over_list() uses apply(), which doesn't work as intended when the array has more than two elements:

alist <- list(
  matrix(1:20, 4),
  matrix(21:40, 4),
  matrix(41:60, 4)
)

# This returns an array, not the average
apply(
    simplify2array(alist),
    seq_along(alist),
    stat
)

# , , 1
# 
# [,1] [,2] [,3] [,4] [,5]
# [1,]    1    5    9   13   17
# [2,]    2    6   10   14   18
# [3,]    3    7   11   15   19
# [4,]    4    8   12   16   20
# 
# , , 2
# 
# [,1] [,2] [,3] [,4] [,5]
# [1,]   21   25   29   33   37
# [2,]   22   26   30   34   38
# [3,]   23   27   31   35   39
# [4,]   24   28   32   36   40
# 
# , , 3
# 
# [,1] [,2] [,3] [,4] [,5]
# [1,]   41   45   49   53   57
# [2,]   42   46   50   54   58
# [3,]   43   47   51   55   59
# [4,]   44   48   52   56   60

I misunderstood the way apply() works with arrays as opposed to dataframes/lists. The loop index shouldn't be one-dimensional (i.e. seq_along(alist)), but two-dimensional- specifically, it should be 1:2:

mean_array <- apply(
  simplify2array(alist),
  1:2,
  stat
)

mean_list <- Reduce(`+`, alist) / length(alist)

# TRUE
identical(
  mean_array,
  mean_list
)

I'll fix this as part of this PR.