Smarter time management in pset.execute() loop

[erikvansebille]

The time management in the pset.execute() loop can be made a bit smarter. For example

1. If the pset is empty, execution can in principle be stopped. Since we don't have a possibility to add particles at runtime (except if repeatdt is used), a run with an empty pset can be stooped. This would also avoid the warning (in particlefile that "ParticleSet is empty on writing as array at time")
2. If there are no particle to be executed in the next loop, that loop can be fast-forwarded. For example, if the time counter is at t=3 (with a dt=1) and the next particle to start is at t=20, then the time can directly be moved to t=20. This saves I/O time for loading in unnecessary Field data