[Experimental] Second attempt toying around with data.table

[JanMarvin]

This looks quicker on my desktop, especially for the update part. IIRC before the update I saw 3.x secs in the initial writing part and 11.x secs in the update part. With this PR 3.x secs in the initial part and 4.x secs in the update part.

library(openxlsx2)

rm(list=ls())
gc()

m <- 1e2
n <- 1e4
mm <- matrix(rnorm(n * m), n, m)
tmm <- t(mm)

invisible(gc())
wb <- wb_workbook()$add_worksheet()
beg <- Sys.time()
wb$add_data(x = mm)
end <- Sys.time()
print(end - beg)

beg <- Sys.time()
wb$add_data(x = mm)
end <- Sys.time()
print(end - beg)