Use of existing cluster for combine preds

Same as my last PR (the one I closed) but this time using sfGetCluster to pass the snow cluster into clusterR. 
Tested and works (combinePreds execution cut to <6 mins).
Here is the full break down of cumulative run time (in seconds, full size grid, 5k occurrences):
```
start 0
starting bhatt 46.405
finished bhatt 6169.5
starting brt 6169.521
finished brt 13553.242
starting combine 14342.247
finshed combine 14692.064
done 14772.804
```