Betweenness: remove some superfluous copies and initializations
Betweenness: remove some superfluous copies and initializations

The thread-local vectors are no longer copied for writing the values
back into the global vector and the thread-local vectors for edge
centralities are only initialized when they are actually needed.

fmaschler/networkit,fmaschler/networkit,fmaschler/networkit,fmaschler/networkit,fmaschler/networkit,fmaschler/networkit