wpilibc: Add overloads for units (#1815)

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.

wpilibc/src/main/native/cpp/IterativeRobotBase.cpp

@@ -27,6 +27,10 @@ IterativeRobotBase::IterativeRobotBase(double period)
     : m_period(period),
       m_watchdog(period, [this] { PrintLoopOverrunMessage(); }) {}
 
+IterativeRobotBase::IterativeRobotBase(units::second_t period)
+    : m_period(period.to<double>()),
+      m_watchdog(period.to<double>(), [this] { PrintLoopOverrunMessage(); }) {}
+
 void IterativeRobotBase::RobotInit() {
   wpi::outs() << "Default " << __FUNCTION__ << "() method... Override me!\n";
 }