Moves all CommandBase functionality into Command and deprecates CommandBase for removal.
Moves all SubsystemBase functionality into Subsystem and deprecates SubsystemBase for removal.
Adds a function to CommandScheduler to remove all registered Subsystems.
Fixes several cases where calling scheduler operations from a command callback could result in NPEs or other issues.
Co-authored-by: Tyler Veness <calcmogul@gmail.com>
