A lot of these are breaking changes. frc::Timer was replaced with the
contents of frc2::Timer. The others were in-place argument changes or
removing deprecated non-unit overloads.
The ranges and which value was specified as highest were incorrect on
some of them. On Linux, the range is 1 to 99 with 99 being highest.
From `man 7 sched`:
```
Processes scheduled under one of the real-time policies (SCHED_FIFO,
SCHED_RR) have a sched_priority value in the range 1 (low) to 99 (high).
```
Also clean up the relevant javadoc and doxygen comments.
Using this overload makes the thread backing the Notifier run at
real-time priority. This improves scheduling jitter substantially (5ms
+- 2ms down to 5ms +- 1ms).
A version isn't provided for Java because making threads real-time can
cause GC deadlocks.
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.
Using std::function<void()> directly makes it much clearer to the user
what kind of function Notifier expects. The Doxygen comments already say
what the function is used for, so the typedef just discards useful
information.
The old headers were moved into folders because doing so avoids polluting
the system include directories.
Folder names were also normalized to lowercase.
