Color-coding of points in layers

[astrofrog]

Similar to #137, it would be nice to be able to be able to define the color-coding of points in layers by specifying a column (currently colorMapColumn) but also colorMapMin and colorMapMax such that:

colorFloat = (parseFloat(row[this.colorMapColumn]) - this.colorMapMin) / (this.colorMapMax - this.colorMapMin);

and then colorFloat would get mapped to a colormap. Until this is present in the JS, we will need to do this in Python and then update the data every time colorMapMin and colorMapMax are changed.