Reimplement pcu::Time with `std::chrono::monotonic_clock`

[bobpaw]

We should just use std::chrono::monotonic_clock? That way the implementation will be the same for both MPI builds and SCOREC_NO_MPI builds

Originally posted by @jacobmerson in https://github.com/SCOREC/core/pull/425#discussion_r1962809721

[cwsmith]

I agree. That is a good idea.