ekf2: baro bias publish minor cleanup

- naming consistency (estimator prefix as "namespace")
 - only publish if baro is available and bias is changing as a small logging optimization
 - avoid unnecessary copying (get const reference to status directly)
 - trivial code style fixes

msg/estimator_baro_bias.msg

@@ -0,0 +1,10 @@
+uint64 timestamp                # time since system start (microseconds)
+uint64 timestamp_sample         # the timestamp of the raw data (microseconds)
+
+uint32 baro_device_id		# unique device ID for the sensor that does not change between power cycles
+float32 bias			# estimated barometric altitude bias (m)
+float32 bias_var		# estimated barometric altitude bias variance (m^2)
+
+float32 innov			# innovation of the last measurement fusion (m)
+float32 innov_var		# innovation variance of the last measurement fusion (m^2)
+float32 innov_test_ratio	# normalized innovation squared test ratio