Make channel back into a unique integer value

[keomabrun]

The current channels column requires extra parsing function and is not useful in practice.
The channel is either known or unknown.

For instance:

• in Mercartor, the channel in know so the value with be for instance 11
• in SMIP, the channel value in unknown so the value will be empty (no value between to commas)

This new format does not allow to group multiple channels (when multiple channels have the same RSSI value for instance) but that is rarely the case and the grouping (compression) does not bring a huge benefice.

If the channel is unknown, then the script that use the trace (e.g the 6tisch simulator) can set all the channels to the same value.

The channels that were used during the trace collection are supposed to be provided in the JSON header.

This reverts #2