Only simulate if particle was alive

Only simulate if particle was alive

When spliting the simulation into multiple parts, do not simulate for
times before the particle was born. Otherwise huge simulation times lead
to huge effects even on particles with a simulated birth of near the end
of that time.

Change-Id: I7ff507b27bf819dbebd61934b9523b23316b5e80
Reviewed-by: Martin Jones <martin.jones@nokia.com>