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).

**Update**: the last checkpoint/notify time step is now remembered by the plugin itself and can be retrieved by calling `getLastCheckpoint()`/`getLastNotify()`.

@f-schmitt-zih This should not affect any stuff related to HDF5/ADIOS, right?

@PrometheusPi Will the radiation restart still work with that change?
this function should be `const`