It defaults to .01 and must be in the interval ]0, 1]
If a message expands to N objects, then it costs N / group_method_cost
and activity tool stop reserving messages the sum of all costs exceeds 1.

By default, this preserves original behaviour, i.e. MAX_GROUPED_OBJECTS = 100

Value 1 is not rejected to allow generic coding
(ex: activity_kw['group_method_cost'] = 1. / packet_size)