bugfix for msmbuilder issue #247 regarding loading prepared trajectories...

.... If the metric is RMSD or a hybrid metric, then just the trajectories are loaded. But I needed to add a check for LPRMSD, buuuut this doesn't work since it's a plugin. So, the change is to only load prepared trajectories if the metric is in an instance of a subclass of Vectorized. This should work because then we can use np.concatenate on the prepared trajectories. All other metrics are handled by loading trajectories and preparing later.