Add timers to measure CPU time

[yardasol]

Description

Currently, OpenMC only reports wall-clock times. For users wishing to obtain CPU times for scaling studies and other purposes, they must rely on an external timer. It would be nice if OpenMC had this capability internally.

For wall-clock times, it looks like OpenMC uses the <chrono> library. It looks like for getting cputime, we'll want to use the clock() function in the <ctime> library. We will also need to perform some kind of MPI_Reduce to collect cputimes across all MPI ranks.

Alternatives

N/A

Compatibility

There should be no change to the existing API other than a new dataset in statepoint file.