add a reduce X bytes Y flops action

once the root has received all the contributions of X bytes from the
other processes, it computes Y flops. The other processes exit the
function as soon as they their send is completed.

git-svn-id: svn+ssh://scm.gforge.inria.fr/svn/simgrid/simgrid/trunk@6825 48e7efb5-ca39-0410-a469-dd3cf9ba447f