Add Munsell palette

The Munsell color system is described in detail at https://en.wikipedia.org/wiki/Munsell_color_system This commit adds a new palette to share/palettes that contains a Munsell Color chart in the GIMP .gpl format. The palette is organized by hue (the 40 basic hues in the Munsell system). Within each hue, there are 9 value/chroma gradients and displays as formatted hue pages in the Swatches grid view as per the recommendation of Marc Jeanmougin.