Warning when particles are initialised outside time domain

[VeckoTheGecko]

@sruehs was encountering a bug with her simulation where the number of the particles in the output weren't the same as in the particle set. Turns out not all particles in the particleset were initialised within the time domain of pset.execute(runtime=...). Having an informative warning here would be helpful. Note that the warning would be raised if pset.execute() is used in a loop, but that can now be silenced with a filter from warnings.

cc @michaeldenes