Improves the APIs for various prebuilt subsystems (PIDSubsystem, TrapezoidProfileSubsystem, ProfiledPIDSubsystem). Addresses #2128, and also changes the rather cumbersome getSetpoint API to a more intuitive setSetpoint one. Updates examples to match.
This is the C++ version of #1682.
The old command framework is still available, but will be deprecated.
Due to name conflicts, the new framework is in the frc2 namespace.
Eventually (after the old command framework is removed in a future year)
it will be moved into the main frc namespace.
