CPU_TIME reduction

[alwinm]

Currently the time reduction uses MPI_Allreduce simply because some reduction routines already existed in mpi_routines.h

This performs an extra unnecessary step of sending the result back out to all procs, but I believe only proc 0 needs the reduced data. MPI_Reduce should suffice.

[bcaddy]

With some refactoring this could also be an asynchronous call, no need for the result until the next time that timer is called.

[bcaddy]

FYI, the average reduction OneTime::End actually does need to be an allreduce since it's used to check if that rank is particularly slow.