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.