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/Timer.h

@@ -7,6 +7,7 @@
 
 #pragma once
 
+#include <units/units.h>
 #include <wpi/deprecated.h>
 #include <wpi/mutex.h>
 
@@ -101,8 +102,19 @@ class Timer {
    * @param period The period to check for (in seconds).
    * @return       True if the period has passed.
    */
+  WPI_DEPRECATED("Use unit-safe HasPeriodPassed method instead.")
   bool HasPeriodPassed(double period);
 
+  /**
+   * Check if the period specified has passed and if it has, advance the start
+   * time by that period. This is useful to decide if it's time to do periodic
+   * work without drifting later by the time it took to get around to checking.
+   *
+   * @param period The period to check for.
+   * @return       True if the period has passed.
+   */
+  bool HasPeriodPassed(units::second_t period);
+
   /**
    * Return the FPGA system clock time in seconds.
    *