Categoricals in scatter

[jfoster17]

Allow Categorical Variables to be Used in Color and Size Scaling in Scatter viewer

Description

This allows scatter plot cmap and size scaling attributes to be a categorical. This does not include a qualitative color-map for categorical attributes (as proposed in #1689) and obviously the size scaling is, in general, not going to be very meaningful, but for a small number of categorical levels it helps makes the points distinct.

An unresolved problem with this is that it might break glue-jupyter scatter plot because that does not consistently call ensure_numerical on the relevant attributes.