Allow searcK to detect multiple cores

[marineleroi]

By default searchK() uses only one core. If one wants to use all available cores, they have to explicitly specify cores = parallel::detectCores() which is inconvenient because the underlying parallel::mclapply() function already seems to do that when the value is null.

Passing cores = NULL ends up with the following error on version 1.3.6:

searchK(out$documents, out$vocab, K=c(10:20), data=meta, cores = NULL)
Error in if (cores == 1) { : argument is of length zero

It would be convenient to allow the above snippet to work.