Process a dataframe in canonical form ``(t_step, agent_id, key, value, value_type)`` into
    a dataframe with a column per key

def process_one(df, *keys, columns=['key', 'agent_id'], values='value',
                fill=True, index=['t_step',],
                aggfunc='first', **kwargs):
    '''
    Process a dataframe in canonical form ``(t_step, agent_id, key, value, value_type)`` into
    a dataframe with a column per key
    '''
    if df is None:
        return df
    if keys:
        df = df[df['key'].isin(keys)]

    df = df.pivot_table(values=values, index=index, columns=columns,
                        aggfunc=aggfunc, **kwargs)
    if fill:
        df = fillna(df)
    return df