Centralize management of forces ready on GPU event

Centralize management of forces ready on GPU event

This change adds the GpuEventSynchronizer for the forces reduced on GPU event
to the StatePropagatorDataGpu. This event should be marked if the buffer ops
are offloaded when the force reduction is done. The consumers of of the forces
on the GPU will get this event or the event on the H2D copy is done,
depending on the current step workload and offload scenario.

Change-Id: Ib559dbed5ad777eac3a906e4ee0ebaa07caf0ac1