Will enable/disable multi-threaded reading or writing
at C++ level.
     
    
    Usage
    filearray_threads(n, ...)
 
     
    
    Arguments
- n
 
number of threads to set. If n is negative,
then default to the number of cores that computer has.
 
- ...
 
internally used
 
 
    
    Value
    An integer of current number of threads