Callbacks
n A generic way to transfer control back to a
client after a library has finished.
n After finishing a reduction, the results
have to be passed to some chare's entry
method.
n To do this, create an object of type
CkCallback with chare's ID & entry
method index
n Different types of callbacks
n One commonly used type:
     CkCallback cb(<chare’s entry method>,<chare’s proxy>);