a bool to a enum osgDB::Registru::CacheHintOptions to be used a bit mask flag.
From Robert Osfied, added osgDB::Registry::ReadFileCallback and
osgDB::Registry::WriteFileCallback to allow customizition of read and write
calls.
leaving it up to the Viewer to specify the mode (which by default is MultiThreaded).
Added a check for the presence of osgParticle systems so that threading is
disabled in this case.
