The device might not always send its periodicity to the network-server
(using mac-commands). As well there is some ambiguity about the default
ping-slot data-rates. While the Regional Parameters Specification
defines the default beacon data-rates, it only defines the default
ping-slot frequency for Class-B.
This also changes the API field from class_b_ping_slot_period to
class_b_ping_slot_nb_k, where ..._k must be between 0 - 7 as defined by
the LoRaWAN Specification. This removes some ambiguity as 'period' could
mean different things in different contexts.
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