[wpiutil] Split units.h into separate headers for each unit (#2551)

Closes #2508.

Co-authored-by: Prateek Machiraju <prateek.machiraju@gmail.com>

wpilibcExamples/src/main/cpp/examples/ElevatorTrapezoidProfile/cpp/Robot.cpp

@@ -1,5 +1,5 @@
 /*----------------------------------------------------------------------------*/
-/* Copyright (c) 2019 FIRST. All Rights Reserved.                             */
+/* Copyright (c) 2019-2020 FIRST. All Rights Reserved.                        */
 /* Open Source Software - may be modified and shared by FRC teams. The code   */
 /* must be accompanied by the FIRST BSD license file in the root directory of */
 /* the project.                                                               */
@@ -9,7 +9,11 @@
 #include <frc/TimedRobot.h>
 #include <frc/controller/SimpleMotorFeedforward.h>
 #include <frc/trajectory/TrapezoidProfile.h>
-#include <units/units.h>
+#include <units/acceleration.h>
+#include <units/length.h>
+#include <units/time.h>
+#include <units/velocity.h>
+#include <units/voltage.h>
 #include <wpi/math>
 
 #include "ExampleSmartMotorController.h"
@@ -33,8 +37,8 @@ class Robot : public frc::TimedRobot {
     // Create a motion profile with the given maximum velocity and maximum
     // acceleration constraints for the next setpoint, the desired goal, and the
     // current setpoint.
-    frc::TrapezoidProfile<units::meter> profile{m_constraints, m_goal,
-                                                m_setpoint};
+    frc::TrapezoidProfile<units::meters> profile{m_constraints, m_goal,
+                                                 m_setpoint};
 
     // Retrieve the profiled setpoint for the next timestep. This setpoint moves
     // toward the goal while obeying the constraints.