Return an array with the trace of all stochastics, sliced by sl.

def trace2array(self, sl):
        """Return an array with the trace of all stochastics, sliced by sl."""
        chain = []
        for stochastic in self.stochastics:
            tr = stochastic.trace.gettrace(slicing=sl)
            if tr is None:
                raise AttributeError
            chain.append(tr)
        return np.hstack(chain)