PMD requires that variables only initialized in the constructor be
final. The compiler errors if those final variables aren't guaranteed to
be initialized, so extra else branches were added to ensure that.
PMD also requires that classes with only private constructors be final.
The equivalent C++ classes were finalized as well, except for
TimeInterpolatableBuffer because it doesn't expose factory functions.
The move ctor is trying to cast from e.g. SendableHelper to PIDController before PIDController has been constructed, which is potentially UB. We don't actually use anything in PIDController though, so it's OK in our case.
The non-NT portion has been moved to wpiutil.
The NT portion has been moved to ntcore (as NTSendable).
SendableBuilder similarly split and moved.
SendableRegistry moved to wpiutil.
In C++, SendableHelper also moved to wpiutil.
This enables use of Sendable from wpimath and also enables
moving several classes from wpilib to wpimath.
