multi-combine-randomforest

# things that affect speeds of random forests
# http://stackoverflow.com/questions/14106010/parallel-execution-of-random-forest-in-r/15771458#15771458
# also
# http://stackoverflow.com/questions/23075506/how-to-improve-randomforest-performance?lq=1
# also
#

Setting .multicombine to TRUE can make a significant difference:
````R
rf <- foreach(ntree=rep(25000, 6), .combine=combine, .multicombine=TRUE,
              .packages='randomForest') %dopar% {
    randomForest(x, y, ntree=ntree)
}
````
This causes combine to be called once rather than five times. On my desktop machine, this runs in 8 seconds rather than 19 seconds.