add FlyTo path functionality

We don't currrently support flyTo entity paths, but it's come up on the forum a few times. We should consider adding it as an option if there's not a strong reason against it.

https://groups.google.com/forum/?hl=en#!searchin/cesium-dev/path%7Csort:date/cesium-dev/A01euOdSy3A/ZoZZviNxCwAJ

Start with the linked Cesium forum discussion and inspect the existing flyTo and entity-path entry points to understand the requested behavior. Define what supporting flyTo for entity paths as an option would mean, including how completion can be verified; the issue is done when that behavior is implemented and covered by appropriate tests.

Written by the indexing model from the issue text.