AP_NavEKF: add enumeration to document EKF SolutionStatus

this isn't used for anything but documenting the solution status field, which can be used in the Wiki and in various log analysis tools

libraries/AP_NavEKF/AP_Nav_Common.h

@@ -19,6 +19,30 @@
 #include <stdint.h>
 #include <AP_Math/AP_Math.h>
 
+// enumeration corresponding to buts within nav_filter_status union.
+// Only used for documentation purposes.
+enum class NavFilterStatusBit {
+    ATTITUDE           =  0, // 0 - true if attitude estimate is valid
+    HORIZ_VEL          =  1, // 1 - true if horizontal velocity estimate is valid
+    VERT_VEL           =  2, // 2 - true if the vertical velocity estimate is valid
+    HORIZ_POS_REL      =  3, // 3 - true if the relative horizontal position estimate is valid
+    VERT_POS_REL       =  4, // 4 - true if the absolute horizontal position estimate is valid
+    VERT_POS           =  5, // 5 - true if the vertical position estimate is valid
+    TERRAIN_ALT        =  6, // 6 - true if the terrain height estimate is valid
+    CONST_POS_MODE     =  7, // 7 - true if we are in const position mode
+    PRED_HORIZ_POS_REL =  8, // 8 - true if filter expects it can produce a good relative horizontal position estimate - used before takeoff
+    PRED_HORIZ_POS_ABS =  9, // 9 - true if filter expects it can produce a good absolute horizontal position estimate - used before takeoff
+    TAKEOFF_DETECTED   = 10, // 10 - true if optical flow takeoff has been detected
+    TAKEOFF_EXPECTED   = 11, // 11 - true if filter is compensating for baro errors during takeoff
+    TOUCHDOWN_EXPECTED = 12, // 12 - true if filter is compensating for baro errors during touchdown
+    USING_GPS          = 13, // 13 - true if we are using GPS position
+    GPS_GLITCHING      = 14, // 14 - true if GPS glitching is affecting navigation accuracy
+    GPS_QUALITY_GOOD   = 15, // 15 - true if we can use GPS for navigation
+    INITALIZED         = 16, // 16 - true if the EKF has ever been healthy
+    REJECTING_AIRSPEED = 17, // 17 - true if we are rejecting airspeed data
+    DEAD_RECKONING     = 18, // 18 - true if we are dead reckoning (e.g. no position or velocity source)
+};
+
 union nav_filter_status {
     struct {
         bool attitude           : 1; // 0 - true if attitude estimate is valid