Add ::selectSubBand() to enable working with 8 Channel gateways. Add ::setDrTxPow() to allow setting a specific Data Rate and Transmit Power when not using ADR mode.

[dbowerman]

Add ::selectSubBand() to enable working with 8 Channel gateways. Add ::setDrTxPow() to allow setting a specific Data Rate and Transmit Power when not using ADR mode.

[manuelbl]

I've decided to go with a slightly different implementation:

• Sub-band 2 is selected by default (for regions with sub-bands). So it's usually not necessary to select it explicitly.
• If needed, there are methods to select the sub-band, data rate and max. TX power

The changes are currently in the master branch. I would appreciate it if you could give it a test.