Plugins: Notify before Checkpoint

A plugin's `notify()` method is now called *before* it creates a checkpoint.

Corresponds to the problems encountered [here](https://github.com/ComputationalRadiationPhysics/picongpu/pull/540#issuecomment-54168392).

@f-schmitt-zih This should not affect any stuff related to HDF5/ADIOS, right?

@PrometheusPi Will the radiation restart still work with that change?
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