Remove the earliest n ensemble members from the MCMC output

def burnin(self, n):
        """Remove the earliest n ensemble members from the MCMC output"""
        self.sediment_rate = self.sediment_rate[:, n:]
        self.headage = self.headage[n:]
        self.sediment_memory = self.sediment_memory[n:]
        self.objective = self.objective[n:]