Add Waveform Chain command

[adrienmo]

You can compose more complex waveform by chaining them.
For example, pulsing a GPIO on and off every 500 ms for 10 times and then 50 ms for 100 times and this forever. Chain can be nested.

pulses_500 = [
  %Pigpiox.Waveform.Pulse{gpio_on: gpio, delay: 500_000},
  %Pigpiox.Waveform.Pulse{gpio_off: gpio, delay: 500_000}
]

Pigpiox.Waveform.add_generic(pulses_500)

{:ok, wave_500} = Pigpiox.Waveform.create()

pulses_50 = [
  %Pigpiox.Waveform.Pulse{gpio_on: gpio, delay: 50_000},
  %Pigpiox.Waveform.Pulse{gpio_off: gpio, delay: 50_000}
]

Pigpiox.Waveform.add_generic(pulses_50)

{:ok, wave_50} = Pigpiox.Waveform.create()

Pigpiox.GPIO.set_mode(gpio, :output)

Pigpiox.Waveform.chain(%Pigpiox.Waveform.ChainElement{
  content: [
    %Pigpiox.Waveform.ChainElement{
      content: [wave_500],
      repeat: 10
    },
    %Pigpiox.Waveform.ChainElement{
      content: [wave_50],
      repeat: 100
    }
  ],
  repeat: :forever
})

Pigpiox.Waveform.repeat(wave_id)