Default to 2 threads but show a startup message

[mnwright]

No description provided.

[mnwright]

Implements #513.

[mnwright]

Examples:

library(ranger)

ranger 0.16.0 using 2 threads (default). Change with num.threads in ranger() and predict(), options(Ncpus = N), options(ranger.num.threads = N) or environment variable R_RANGER_NUM_THREADS.

options(ranger.num.threads = 4)
library(ranger)

ranger 0.16.0 using 4 threads as set by options(ranger.num.threads = N). Can be overwritten with num.threads.

[mnwright]

Now options(Ncpus = 4) also works.

Precedence:

1. num.threads in ranger/predict call
2. Environment variable R_RANGER_NUM_THREADS
3. options(ranger.num.threads = N)
4. options(Ncpus = N)