Create examples for all classes

[chapulina]

Desired behavior

Users learning about Ignition Math should be able to refer to simple examples to learn how to use the APIs. The closes to that we have are the unit tests. We should have examples in C++, as well as any scripting languages that we support (currently Ruby and Python).

Alternatives considered

Tell users to look at unit tests.

Implementation suggestion

Add examples to the examples folder. The example should then be used as a doxygen \snippet in the class's doxygen description. For example:

https://github.com/ignitionrobotics/ign-math/blob/d338701cd27c172aa931634335024f4c68c78e2c/include/ignition/math/Angle.hh#L60

See how it's rendered here: https://ignitionrobotics.org/api/math/6.9/classignition_1_1math_1_1Angle.html#details

Each example can be added in a separate pull request. This is a great issue for first-time contributors!

Here's the status:

1. Angle
2. AxisAlignedBox
3. Box
4. Color Add example and modify document for class Color #304
5. Cylinder
6. DiffDriveOdometry Add DiffDriveOdometry pybind11 interface and examples #314
7. Filter
8. Frustum
9. GaussMarkovProcess Add GaussMarkovProcess pybind11 interface and examples #315
10. graph/Edge
11. graph/GraphAlgorithms
12. graph/Graph
13. graph/Vertex
14. Helpers
15. Inertial
16. Kmeans kmeans examples in cpp and python #356
17. Line2
18. Line3
19. MassMatrix3
20. Material
21. MaterialType
22. Matrix3
23. Matrix4
24. MovingWindowFilter
25. OrientedBox
26. PID
27. Plane
28. Pose3
29. Quaternion pull request #327
30. Rand Added Rand pybind11 interface and example #312
31. RollingMean
32. RotationSpline
33. SemanticVersion
34. SignalStats
35. Sphere
36. SphericalCoordiantes
37. Spline
38. Stopwatch
39. Temperature
40. Triangle3
41. Triangle
42. Vector2
43. Vector3
44. Vector3Stats
45. Vector4

Additional context

This was broken off #101