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/splinedrivetrain.cc b/frc971/control_loops/drivetrain/splinedrivetrain.cc
index 977529f..508967c 100644
--- a/frc971/control_loops/drivetrain/splinedrivetrain.cc
+++ b/frc971/control_loops/drivetrain/splinedrivetrain.cc
@@ -11,9 +11,7 @@
 #include "frc971/control_loops/drivetrain/drivetrain_output_generated.h"
 #include "frc971/control_loops/drivetrain/drivetrain_status_generated.h"
 
-namespace frc971 {
-namespace control_loops {
-namespace drivetrain {
+namespace frc971::control_loops::drivetrain {
 
 SplineDrivetrain::SplineDrivetrain(const DrivetrainConfig<double> &dt_config)
     : dt_config_(dt_config),
@@ -268,6 +266,4 @@
   return MakeTrajectoryLogging(builder->fbb());
 }
 
-}  // namespace drivetrain
-}  // namespace control_loops
-}  // namespace frc971
+}  // namespace frc971::control_loops::drivetrain