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/include/frc/Notifier.h

@@ -15,6 +15,8 @@
 #include <utility>
 
 #include <hal/Types.h>
+#include <units/units.h>
+#include <wpi/deprecated.h>
 #include <wpi/mutex.h>
 
 #include "frc/ErrorBase.h"
@@ -58,8 +60,18 @@ class Notifier : public ErrorBase {
    *
    * @param delay Seconds to wait before the handler is called.
    */
+  WPI_DEPRECATED("Use unit-safe StartSingle method instead.")
   void StartSingle(double delay);
 
+  /**
+   * Register for single event notification.
+   *
+   * A timer event is queued for a single event after the specified delay.
+   *
+   * @param delay Amount of time to wait before the handler is called.
+   */
+  void StartSingle(units::second_t delay);
+
   /**
    * Register for periodic event notification.
    *
@@ -70,8 +82,21 @@ class Notifier : public ErrorBase {
    * @param period Period in seconds to call the handler starting one period
    *               after the call to this method.
    */
+  WPI_DEPRECATED("Use unit-safe StartPeriodic method instead.")
   void StartPeriodic(double period);
 
+  /**
+   * Register for periodic event notification.
+   *
+   * A timer event is queued for periodic event notification. Each time the
+   * interrupt occurs, the event will be immediately requeued for the same time
+   * interval.
+   *
+   * @param period Period to call the handler starting one period
+   *               after the call to this method.
+   */
+  void StartPeriodic(units::second_t period);
+
   /**
    * Stop timer events from occuring.
    *