set_sample_group -- return n_configured, make `chans` argument type `int | None` where `None` means "ALL"

[cboulay]

n_configured = session.set_sample_group(                  
      chans=None,                  # None = all matching                                                                                                
      channel_type=ChannelType.FRONTEND,
      rate=SampleRate.SR_2kHz,                                                                                                                                                  
      disable_others=True,                                  
  )       

Knowing how many channels were successfully configured simplifies some other operations like channel map auto-filling.

[cboulay]

Closed with #182

But ultimately we abandoned this idea because it slowed down the interaction too much.