Topic plugins restart

Another pull to implement issue #114.

This pull starts to add restarting-capabilities to plugins by defining an interface for restarts and moving HDF5 restart code from (now removed) SimRestartInitialiser to HDF5Writer as plugins should be able to restart themselves. The code from `SimRestartInitialiser` mainly moved to two new files, `RestartFieldLoader` and `RestartParticleLoader`.

It also adds two new command line arguments, `--restart-step` which defines the program-wide timestep to restart from and `--hdf5.restart-file` since the HDF5 restart file only needs to be known by the HDF5Writer plugin, now.

This code has been tested to work with 2 GPUs with moving window so far but should work for other combinations too since the actual loading code has not been altered. I will run additional the required tests in the next days.