Prohibit identity zoom functions

[jfirebaugh]

Our function taxonomy benefits from being pared down where possible. Identity zoom functions don't seem useful, and are not implemented in native.

Fixes #4151.

[jplante]

This is a feature we would like. We think it's useful.

For example - ALA Leaflet's Circle and CircleMarker - where the radius is based on either distance or pixels - we could use this for setting different radius for a Circle based on data identity attributes

[jfirebaugh]

different radius for a Circle based on data identity attributes

That sounds like a regular property-based identity function (which is supported), not an identity zoom function. An identity zoom function would be a function whose output is always the map's current zoom level. (If that's really what you want, you can create an exponential zoom function with base 1 and stops [[0, 0], [maxZoom, maxZoom]], where maxZoom is some arbitrary value >= the maximum zoom your map allows.)