Document and provide options for how size/color scaling is done for layers

[astrofrog]

We should document how the scaling of points and the color-coding works when color-coding by attributes. In particular, do we have control over the min/max values to use for this encoding?

We should also expose options related to these, if any - for example it looks like there is a pointScaleType setting that can control this behavior, so we should investigate how this works. Similarly, there may be ways to customize the colormap to use.