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 +19,29 @@
 #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           =     1, // attitude estimate valid
+    HORIZ_VEL          =     2, // horizontal velocity estimate valid
+    VERT_VEL           =     4, // vertical velocity estimate valid
+    HORIZ_POS_REL      =     8, // relative horizontal position estimate valid
+    VERT_POS_REL       =    16, // absolute horizontal position estimate valid
+    VERT_POS           =    32, // vertical position estimate valid
+    TERRAIN_ALT        =    64, // terrain height estimate valid
+    CONST_POS_MODE     =    128, // expected good relative horizontal position estimate - used before takeoff
+    PRED_HORIZ_POS_ABS =    512, // expects good absolute horizontal position estimate - used before takeoff
+    TAKEOFF_DETECTED   =   1024, // optical flow takeoff has been detected
+    TAKEOFF_EXPECTED   =   2048, // compensating for baro errors during takeoff
+    TOUCHDOWN_EXPECTED =   4096, // compensating for baro errors during touchdown
+    USING_GPS          =   8192, // using GPS position
+    GPS_GLITCHING      =  16384, // GPS glitching is affecting navigation accuracy
+    GPS_QUALITY_GOOD   =  32768, // can use GPS for navigation
+    INITALIZED         =  65536, // has ever been healthy
+    REJECTING_AIRSPEED = 131072, // rejecting airspeed data
+    DEAD_RECKONING     = 262144, // dead reckoning (e.g. no position or velocity source)
+};
+
 union nav_filter_status {
     struct {
         bool attitude           : 1; // 0 - true if attitude estimate is valid