Arm works

Added gravity and calibrated it.  Terifying...

Change-Id: I70babb1cd3b83ddd7a81f06fb2a75cefd55bcdb8
diff --git a/y2018/control_loops/superstructure/arm/dynamics.cc b/y2018/control_loops/superstructure/arm/dynamics.cc
index 87d2178..02a2861 100644
--- a/y2018/control_loops/superstructure/arm/dynamics.cc
+++ b/y2018/control_loops/superstructure/arm/dynamics.cc
@@ -1,5 +1,7 @@
 #include "y2018/control_loops/superstructure/arm/dynamics.h"
 
+DEFINE_bool(gravity, true, "If true, enable gravity.");
+
 namespace y2018 {
 namespace control_loops {
 namespace superstructure {