Add function type "identity" to color, radius, height stops

# Problem

As a data scientist or developer, I want to build an application where a visualization's "height", "radius", or "color" is encoded directly into the source data as a property - for example, `"height": 2001. This matters because I can create buildings that are the exact correct height in a 3D extrusion visualization.

# Solution

Add `identity` function to https://github.com/mapbox/mapboxgl-jupyter/blob/6e9543462d149118fb81b0c9298bea1dd44a8be2/mapboxgl/viz.py#L589