More explicit options to set nThreads

[adamsardar]

At current there are alot of 'automatic' choices with regards to multithreading. This isn't ideal - ideally we'd allow the user to choose.

Perhaps use options("stoneTrees_nThreads")?

[adamsardar]

Threading is complex issue and different settings can affect performance in wildly different ways. Perhaps this is yet another reason to support #15 - delegate all these options to ROI