(Polygons with 2 points are invalid)

Fix segfault in Memory Viewer when displaying Pose
Fix return type of ConvertToArray()

Revise Predictive: Split into Predictive (base, no children) and PredictiveContainer (with children)

Make ProviderSegment a Predictive, others a PredictiveContainer.
Remove non-const ref access to underlying container

Also move generic templates into detail namespace.

Turns out std::stringstreams can't be copied, so you can't use them in
containers.