keyby should recycle scalar inputs

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@shrektan is already working on this.

Since Apr 29, 2020.

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Description

feature request

Sometimes, I want to have an additional column after keyby. I use dt[, .(V = sum(A)), keyby = .(B, C = "CCC")] and surprisingly find it throws an error. Yes, I know I have many other options. But I feel this way is more natural. In addition, considering the fact that we recycle scalar inputs generally, we should support it for keyby as well.

library(data.table)
dt = data.table(A = 1:10, B = rep(c("A", "B"), length.out = 10))
dt[, .(V = sum(A)), keyby = .(B, C = rep("CCC", 10))]
#>         B      C     V
#>    <char> <char> <int>
#> 1:      A    CCC    25
#> 2:      B    CCC    30

# I expect this works 
dt[, .(V = sum(A)), keyby = .(B, C = "CCC")]
#> Error in `[.data.table`(dt, , .(V = sum(A)), keyby = .(B, C = "CCC")): The items in the 'by' or 'keyby' list are length(s) (10,1). Each must be length 10; the same length as there are rows in x (after subsetting if i is provided).

Created on 2020-04-29 by the reprex package (v0.3.0)

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