A color scale is tied to a single heat map, making it impossible to have multiple heatmaps with the same color scale

It could be fixed by making the color-mapper separate from the heat map, and then either requiring the color-mapper to be an argument to NewHeatMap, or requiring GridXYZ to be a matrix of colors rather than a matrix of floats.

Making these separate would also be useful in other areas, such as allowing plots with points or lines that are colored according to some attribute of the data.

I think this is related to #302.