Halo evaluation as objective

It would be nice having a halo evaluation as an objective for the sampler and optimizer. I also think about multi-bunch simulations. We need either the halo written to the stat-file or evaluate it using the H5 file. In case of multi-bunch simulations where particles are assigned to energy bins, we'd need a stat file for every energy bin. Therefore, I think, it's easier to add new objective funtion.

I suggest something like this

halo(dir, step, bin)

where dir=x,y,z and bin the energy bin (default: 0).