Track gyro using a DCM.

This brings in an implementation of some general matrix manipulation routines,
not all of which are used at teh moment.

Output the DCM on the UART at 50Hz for the host to display it usefully.

dcm.h

@@ -0,0 +1,9 @@
+/* dcm.h */
+
+#include "types.h"
+
+void dcm_update(float omega_x, float omega_y, float omega_z);
+void dcm_normalise(void);
+bool dcm_renormalise(float *v);
+void dcm_dump(void);
+void dcm_send_packet(void);