Takes in an abbreviation for a quantity and a quantity dictionary,
       and returns a more descriptive string of the quantity as "name (units)."
       Raises ValueError if the name is not in quantity_dict

def quantity_string(name, quantity_dict):
    '''Takes in an abbreviation for a quantity and a quantity dictionary,
       and returns a more descriptive string of the quantity as "name (units)."
       Raises ValueError if the name is not in quantity_dict
    '''
    if name not in quantity_dict.keys():
        raise ValueError('{0} is not present in quantity_dict'.format(name))
    return '{0} ({1})'.format(quantity_dict[name]['name'],
                              quantity_dict[name]['units'])