Colormap: shortcut to get a range of colors

This is a common task:

```cs
ScottPlot.Colormaps.Turbo cmap = new();

for (int i = 0; i < count; i++)
{
    double fraction = (double)i / (count - 1);
    sig.Color = cmap.GetColor(fraction);
```

It would be nice if there were a shortcut to get a collection of colors given a start and stop position on a colormap