Support null speed on ReferenceTarget (#11)

* Add a sentinel value for ReferenceTarget::measured_speed_m_s that indicates no measurement is available, so no value is provided

* Update to NO_MEASUREMENT and add usage instructions

Author: emersonknapp

msg/ReferenceTarget.msg

@@ -1,3 +1,9 @@
+# Constant representing that the value in question couldn't be measured
+# This often applied speed or heading/yaw for example in single-antenna GPS
+# For orientation, use NO_MEASUREMENT as the `w` value of the quaternion
+# This value is arbitrary and should be used by name rather than by value, but it's chosen to be exactly-representable in a float32 so there aren't floating point comparison issues
+float32 NO_MEASUREMENT = -9999.5
+
 # PoseStamped data for reference target pose
 geographic_msgs/GeoPoseStamped pose