Add unit-taking overloads to the following classes:
- IterativeRobotBase
- LinearFilter
- Notifier
- TimedRobot
- Timer (HasPeriodPassed only)
- frc2::PIDController
The corresponding non-units-taking functions have been deprecated.
The return value of TimedRobot::GetPeriod() was updated.
This is a breaking change, users should use to<double> to get the value in seconds.
Other return values, e.g. Timer::Get(), have NOT been updated due to much wider use.
Timer didn't have working move semantics because mutexes aren't
moveable, meaning the default implementations were ill-formed.
MotorSafety wasn't locking its mutex.
The old headers were moved into folders because doing so avoids polluting
the system include directories.
Folder names were also normalized to lowercase.
