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/trajectory.cc b/frc971/control_loops/drivetrain/trajectory.cc
index 76a6784..36d0947 100644
--- a/frc971/control_loops/drivetrain/trajectory.cc
+++ b/frc971/control_loops/drivetrain/trajectory.cc
@@ -12,9 +12,7 @@
 #include "frc971/control_loops/hybrid_state_feedback_loop.h"
 #include "frc971/control_loops/state_feedback_loop.h"
 
-namespace frc971 {
-namespace control_loops {
-namespace drivetrain {
+namespace frc971::control_loops::drivetrain {
 
 namespace {
 float DefaultConstraint(ConstraintType type) {
@@ -906,6 +904,4 @@
   return result;
 }
 
-}  // namespace drivetrain
-}  // namespace control_loops
-}  // namespace frc971
+}  // namespace frc971::control_loops::drivetrain