Plate Carre - inconsistent handling of Z values

[gwaldron]

Plate Carre does not have a well-defined mapping for Z/height values. In MapEngine?, we call HeightFieldUtils::scaleHeightFieldtoDegrees() as a simple attempt to do "something" to make it look OK, but it is not a general purpose solution. Really there should be a default behavior and perhaps a callback that lets the user do something custom.

Also, the ObjectPlacer? (et al) needs to respect this settings and act consistently (which it does not do at the moment).