Fix/CRAN n cores notes

[robitalec]

CRAN's recent checks

0 on Windows
3 NOTEs on Debian

* checking examples ... [61s/8s] NOTE
Examples with CPU (user + system) or elapsed time > 5s
* checking tests ... [89s/15s] NOTE
  Running ‘testthat.R’ [89s/15s]
Running R code in ‘testthat.R’ had CPU time 6 times elapsed time
* checking re-building of vignette outputs ... [31s/7s] NOTE
Re-building vignettes had CPU time 4.3 times elapsed time 

Suggested fix:

• "reduce the number of parallel processes/threads"
• "do not use more than 2 cores"

Related:

• Best practice for using max 2 cores on CRAN? Rdatatable/data.table#5658
• https://stat.ethz.ch/pipermail/r-package-devel/2023q3/009454.html
• https://stat.ethz.ch/pipermail/r-package-devel/2023q3/009458.html
• many other packages facing similar issues..

[robitalec]

Passing on CRAN by littering examples, tests and vignettes with data.table::setDTthreads(1)