Add support for counter output

[glopesdev]

The DAQmx counter output API is very useful for generating digital trains for experimental control. It is essentially a self-contained task which requires no input buffer to be provided, so it can be implemented as an observable source returning Unit.

[bruno-f-cruz]

Draft of possible implementation:

public override IObservable<Unit> Generate()
        {
            return Observable.Using(
                () => new Task(),
                task => Observable.Create<Unit>(observer =>
                {
                    task.COChannels.CreatePulseChannelFrequency(
                        counterChannel,
                        "",
                        COPulseFrequencyUnits.Hertz,
                        COPulseIdleState.Low,
                        0, frequency, dutyCycle);
                    if (numberOfSamples.HasValue)
                    {
                        task.Timing.ConfigureImplicit(SampleQuantityMode.FiniteSamples, numberOfSamples.Value);
                    }
                    else
                    {
                        task.Timing.ConfigureImplicit(SampleQuantityMode.ContinuousSamples);
                    }
                    task.Control(TaskAction.Verify);
                    task.Control(TaskAction.Commit);
                    task.Start();

                    task.Done += (sender, e) =>
                    {
                        if (e.Error != null) observer.OnError(e.Error);
                        else observer.OnCompleted();
                    };
                    return Disposable.Create(task.Stop);
                }
            )
            );
        }

[sumiya-kuroda]

I think @RoboDoig managed to implement both CO and CI

[RoboDoig]

@sumiya-kuroda - I have project specific implementations for CO and CI in the vr-abcd as extensions, although they are quite specific for generating continuous frequency pulses in the CO case and reading quadrature encoders in the CI case. The NIDAQ functionality for CO and CI is much broader than this and the Bonsai.DAQmx operators should cover the other use cases of CO and CI channels.