Split a dataframe in two dataframes: one with the history of agents,
    and one with the environment history

def split_df(df):
    '''
    Split a dataframe in two dataframes: one with the history of agents,
    and one with the environment history
    '''
    envmask = (df['agent_id'] == 'env')
    n_env = envmask.sum()
    if n_env == len(df):
        return df, None
    elif n_env == 0:
        return None, df
    agents, env = [x for _, x in df.groupby(envmask)]
    return env, agents