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/calibration_accumulator.cc b/frc971/vision/calibration_accumulator.cc
index 3b0bcc6..d8abd33 100644
--- a/frc971/vision/calibration_accumulator.cc
+++ b/frc971/vision/calibration_accumulator.cc
@@ -21,8 +21,7 @@
 DEFINE_bool(save_valid_only, false,
             "If true, only save images with valid pose estimates");
 
-namespace frc971 {
-namespace vision {
+namespace frc971::vision {
 using aos::distributed_clock;
 using aos::monotonic_clock;
 namespace chrono = std::chrono;
@@ -278,5 +277,4 @@
                 gyro, accel * kG);
 }
 
-}  // namespace vision
-}  // namespace frc971
+}  // namespace frc971::vision