Plot.Add.ScatterLine()

It would be nice to have a helper method which under the hood does this:

```cs
var sp = plt.Add.Scatter(xs, ys);
sp.MarkerSize = 0;
```

It could be used like:
```cs
Plot.Add.ScatterLine(xs, ys);
```

A similar method that adds a scatter point with markers but no line would be nice to have too