It breaks the unit system badly; the tolerance member variable has
different units depending on percent vs absolute. Absolute tolerance is
a lot more natural than percent tolerance anyway.
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.
