Number of cores restricted by number of variables when parallelize='forests'

[stephematician]

When parallelizing by dividing up the trees amongst cores, it doesn't make sense to restrict the number of cores to the number of variables in the data set.

require(missForest)
require(doParallel)

cl <- makeCluster(8)
registerDoParallel(cl)
missForest(iris, parallelize='forests')
stopCluster(cl)

Produces the following output:

Error in missForest(data_, parallelize = "forests") : 
  The number of parallel cores should not exceed the number of variables (p=5)