Move IndexedImageFrame to root namespace.

Author: JimBobSquarePants

src/ImageSharp/Processing/Processors/Quantization/IndexedImageFrame{TPixel}.cs renamed to src/ImageSharp/IndexedImageFrame{TPixel}.cs

@@ -7,8 +7,9 @@
 using SixLabors.ImageSharp.Advanced;
 using SixLabors.ImageSharp.Memory;
 using SixLabors.ImageSharp.PixelFormats;
+using SixLabors.ImageSharp.Processing.Processors.Quantization;
 
-namespace SixLabors.ImageSharp.Processing.Processors.Quantization
+namespace SixLabors.ImageSharp
 {
     /// <summary>
     /// A pixel-specific image frame where each pixel buffer value represents an index in a color palette.