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/vision/visualize_robot.cc b/frc971/vision/visualize_robot.cc
index 0c7a536..d13668e 100644
--- a/frc971/vision/visualize_robot.cc
+++ b/frc971/vision/visualize_robot.cc
@@ -6,8 +6,7 @@
 #include <opencv2/highgui/highgui.hpp>
 #include <opencv2/imgproc.hpp>
 
-namespace frc971 {
-namespace vision {
+namespace frc971::vision {
 
 void VisualizeRobot::SetDefaultViewpoint(int image_width, double focal_length) {
   // 10 meters above the origin, rotated so the camera faces straight down
@@ -106,5 +105,4 @@
   DrawLine(rr_corner, fr_corner, color);
 }
 
-}  // namespace vision
-}  // namespace frc971
+}  // namespace frc971::vision