Nest all namespaces

The compiler supports this now.  We can type less going forward.
No functional changes.

Signed-off-by: Stephan Pleines <pleines.stephan@gmail.com>
Change-Id: I29d6fa4f9aacc0e381f1a7637294db0392466995
diff --git a/frc971/control_loops/drivetrain/drivetrain_states.h b/frc971/control_loops/drivetrain/drivetrain_states.h
index 9ea2518..70ad69d 100644
--- a/frc971/control_loops/drivetrain/drivetrain_states.h
+++ b/frc971/control_loops/drivetrain/drivetrain_states.h
@@ -1,9 +1,7 @@
 #ifndef FRC971_CONTROL_LOOPS_DRIVETRAIN_DRIVETRAIN_STATES_H_
 #define FRC971_CONTROL_LOOPS_DRIVETRAIN_DRIVETRAIN_STATES_H_
 
-namespace frc971 {
-namespace control_loops {
-namespace drivetrain {
+namespace frc971::control_loops ::drivetrain {
 
 enum KalmanState {
   kLeftPosition = 0,
@@ -17,8 +15,6 @@
 
 enum OutputState { kLeftVoltage = 0, kRightVoltage = 1 };
 
-}  // namespace drivetrain
-}  // namespace control_loops
-}  // namespace frc971
+}  // namespace frc971::control_loops::drivetrain
 
 #endif  // FRC971_CONTROL_LOOPS_DRIVETRAIN_DRIVETRAIN_STATES_H_