Support SMASH ensemble mode

[NGoetz]

Especially relevant for potential studies, SMASH also supports ensembles, e.g. events happening in parallel, exchanging fields. Such outputs have a more complex structure:

#!OSCAR2013 particle_lists t x y z mass p0 px py pz pdg ID charge
# Units: fm fm fm fm GeV GeV GeV GeV GeV none none e
# SMASH-3.1-232-g69fc57150
# event 0 ensemble 0 out 469
200 -57.1878 68.3635 -40.5932 0.938 1.10218116 -0.33052567 0.411572346 -0.237318968 2112 2493 0
200 -80.3853 -36.553 47.4137 0.938 1.12779724 -0.493834588 -0.226270747 0.311466784 2212 2543 1

in future versions, we might want to support this.