Returns an instance intended for sharing between
            sections of code which wish to make use of
            threading by performing operations in other
            threads, but which don't mind operations being
            interleaved with those belonging to other
            sections of code. 
 Always returns the same
            instance whenever the method is called. 
 The
            shared pool is created with an initial size as
            specified by the GSIOThreadPoolSize user default
            (zero if there is no such positive integer in the
            defauilts system, however, you can modify that
            using the
            
              -setThreads:
            
            method.