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.

[astrofrog]

Just for the record, this is how WWT currently handles layer colors:

            switch (this.get__colorMap()) {
              case 0:
                pointColor = colorLocal;
                break;
              case 3:
                if (this.get_colorMapColumn() > -1) {
                  pointColor = this._parseColor$1(row[this.get_colorMapColumn()], colorLocal);
                }
                else {
                  pointColor = colorLocal;
                }
                break;
              default:
                break;
            }

So for now we need to include a column with the list of colors (one for each point).