Add double jointed arm dynamics.

Change-Id: I882d23955f4ebd367b01b7506d9957ec6f3eed40
diff --git a/y2018/control_loops/superstructure/arm/BUILD b/y2018/control_loops/superstructure/arm/BUILD
index 2862a6a..cbb443d 100644
--- a/y2018/control_loops/superstructure/arm/BUILD
+++ b/y2018/control_loops/superstructure/arm/BUILD
@@ -20,3 +20,28 @@
         "//third_party/eigen",
     ],
 )
+
+cc_library(
+    name = "dynamics",
+    srcs = [
+        "dynamics.cc",
+    ],
+    hdrs = [
+        "dynamics.h",
+    ],
+    deps = [
+        "//frc971/control_loops:runge_kutta",
+        "//third_party/eigen",
+    ],
+)
+
+cc_test(
+    name = "dynamics_test",
+    srcs = [
+        "dynamics_test.cc",
+    ],
+    deps = [
+        ":dynamics",
+        "//aos/testing:googletest",
+    ],
+)