Restart for Plugin BinEnergyParticles

The old text file is opened, filtered for previous values and pre-pended to the new file during restarts.

The old implementation made a manual copy-and-join of the output necessary (see note <sup>1</sup> in the [wiki](https://github.com/ComputationalRadiationPhysics/picongpu/wiki/PIConGPU-Plugins)).

### To Do

- [ ] perform a run time test (again; after #541 was merged)
what happens if `outFile` failed to open (i.e. `writeToFile` is false). should you still flush?