use epos internal decays for exotic resonances

as discussed in the meeting (https://indico.scc.kit.edu/event/4561/) there is a large difference between epos showers with and without internal decays. decided to switch back to internal decays for epos and only have pythia decays for the long lived hadrons like pions, kaons etc. To preserve the history we may eventually also interface the epos decay routine in a separate process (see #642 ). for showers this ought to be irrelevant.