Reports how many threads the rolling computations would put to work on a
large enough input. The count honors options(RcppRoll.threads)
where that is set, and otherwise defers to the OpenMP runtime
default (typically controlled through the OMP_NUM_THREADS
environment variable). Small inputs are always computed serially, and
results do not depend on the number of threads used.
roll_threads()An integer scalar: the maximum number of threads used, or
NA if the package was compiled without OpenMP support --
useful for checking what an installation from sources ended up with.