Add NSD to trajectory calculation

[walljcg]

I think we used to have this but seems to have been dropped. Would like to add the Net-Square Displacement calculation when we generate a trajectory. The basis for the logic is here:

def calculate_nsd(traj):
    def calculate_nsd(group):
        first_point = group.geometry.iloc[0]
        group['nsd'] = group.geometry.apply(lambda point: 
             point.distance(first_point) ** 2)
        return group

    nsd_df = traj.groupby('groupby_col').apply(calculate_nsd)
return nsd_df

Note that this one of those calculations where it only makes sense in the context of a single subject's dataset and not across multiple subjects.