Czmlpy

* Add forwardExtrapolationType and backwardExtrapolationType to Interpolatable.
* Add DirectionList, Double, Integer, ConicSensor, CustomPatternSensor, RectangularSensor.
* Add agi_conicSensor, agi_customPatternSensor, agi_rectangularSensor, agi_fan, agi_vector.

* Increase time resolution
* Fork poliastro/czml3
* Refactor name to czmlpy

* Add several new properties: `ViewFrom`, `Box`, `Corridor`,
`Cylinder`, `Ellipse`, `Ellipsoid`, `TileSet`, `Wall`
* Add new materials: `PolylineOutlineMaterial`, `PolylineGlowMaterial`,
`PolylineArrowMaterial`, `PolylineDashMaterial`
* Add `Position.cartesianVelocity`, `Billboard.eyeOffset`, and
`Label.pixelOffset`
* Add utilities to create and validate colors: `Color.is_valid`,
`utils.get_color_list`
* Other minor additions and bug fixes

Thanks to all contributors!

- Clément Jonglez
- Eleftheria Chatziargyriou
- Idan Miara
- Joris Olympio
- Juan Luis Cano Rodríguez
- Michael Haberler

* Add `Rectangle` and `RectangleCoordinates`

* Fix packaging

* Fix widget for non-local Jupyter notebook deployments