Add channel.enable_output(is_enabled) method

[sdt]

Hello,

I was using your library to write a fy3200s driver for https://github.com/4x1md/sds1004x_bode

Discovered this through trial and error. Works for both channels:

bg0 - turn channel 1 output off
bg1 - turn channel 1 output on
dg0 - turn channel 2 output off
dg1 - turn channel 2 output on

Thanks for your library. Works great.