multi-threading disabled in Thread::run_queue() if nthread=0
multi-threading disabled in Thread::run_queue() if nthread=0

more specifically, if Thread::number_of_threads() returns 0 (either read
from config file, or via command-line), then execute the functors
serially within a single thread.

MRtrix3/mrtrix3,MRtrix3/mrtrix3,MRtrix3/mrtrix3,MRtrix3/mrtrix3,MRtrix3/mrtrix3,MRtrix3/mrtrix3