Bug fix for event passed to GPU comm features

Bug fix for event passed to GPU comm features

The GPU coordinate halo exchange and coordinate PME-PP transfer were
waiting on an event signalling that the coordinates are available on
the GPU that lives in the NB class, but the new state propagator
feature instead records a different event for this purpose, breaking
the dependency. This change fixes the bug by instead pass the state
propagator event as a dependency to these methods. It needs to be done
every step rather than stored in the class since it can change
depending on the type of step.

Change-Id: I7ea048c6f303192c1310d6b5593227c0ad9a81d0