Switch to system_clock in timer_tree_type module
Switch to system_clock in timer_tree_type module

This update uses the system_clock intrinsic to accurately measure elapsed
wall-clock time, replacing calls to the cpu_time intrinsic. In multi-threaded
environments cpu_time may return the net time consumed by all threads, which
is not generally what is wanted.

nncarlson/petaca,nncarlson/petaca