Rethink the parallel parameter

[nanxstats]

For example, consider having this parameter as a global option instead of a function argument.

• To save repeated typing - set it once and for all.
• This is mostly a side effect not controlled by R but the underlying boosting libraries.

Need to do some research on best practices and decide.

[nanxstats]

Fixed by #23.

Decision: Renamed ncpus to n_threads. Including the n_threads argument as a function argument instead of a global option gives users the flexibility to override the option on a per-call basis, which can be useful in some scenarios. Change the default value to 1.