StateSpaceArm example requires missing LinearSystem::Slice(int)

[auscompgeek]

This currently doesn't type check, and the example is disabled in the tests:

https://github.com/robotpy/examples/blob/242924b3843fdcc6efc2cefa8eac7bfff8b6bc48/StateSpaceArm/robot.py#L59

because LinearSystemId.singleJointedArmSystem returns a LinearSystem<2, 1, 2>.

The upstream examples slice the outputs of this LinearSystem:

https://github.com/wpilibsuite/allwpilib/blob/0695a4db8912a65f175c814c30075cc3e87b313f/wpilibcExamples/src/main/cpp/examples/StateSpaceArm/cpp/Robot.cpp#L48-L51

https://github.com/wpilibsuite/allwpilib/blob/0695a4db8912a65f175c814c30075cc3e87b313f/wpilibjExamples/src/main/java/edu/wpi/first/wpilibj/examples/statespacearm/Robot.java#L64