Tawhiri/altitude models

Idea: Multiple altitude models

Currently, the predictor has only one option: ascent at a fixed rate, then descent using a simple model that assumes terminal velocity.

We could have:

• Ascent/Descent (as above)
• Floating balloons (this is an oft-requested feature)
• ???

Generally, it would be nice if it was easy to add new models to the predictor; some sort of base class/subclass thing.