WISH: It should be possible to adjust the number of assigned cores

_2016-11-03: Issue was originally on 'availableCores("mc.cores") should returngetOptions("mc.cores") + 1L' but it recently turned into a more general discussion on how to maximize core utilization. See below._

`future::availableCores("mc.cores")` should really return `getOptions("mc.cores") + 1L`, because from `help('options')`:

> `mc.cores`:
> a integer giving the maximum allowed number of _additional_ R processes allowed to be run in parallel to the current R process. Defaults to the setting of the environment variable `MC_CORES` if set. Most applications which use this assume a limit of 2 if it is unset.

Further clarification: Multicore processing is not supported, it would effectively correspond to `options(mc.cores = 0)` and in that case `availableCores()` returns 1.