AP_Scripting: add mount-poi md

libraries/AP_Scripting/applets/mount-poi.md

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+# Mount POI
+
+This script displays the location (lat, lon and altitude) that the gimbal is currently pointing towards
+
+# Parameters
+
+POI_DIST_MAX : POI's max distance (in meters) from the vehicle
+
+# How To Use
+
+1. Set RCx_OPTION to 300 (scripting1) to allow triggering the POI calculation from an auxiliary switch
+2. Optionally set POI_DIST_MAX to the maximum distance (in meters) that the POI point could be from the vehicle
+3. Fly the vehicle and point the camera gimbal at a point on the ground
+4. Raise the RC auxiliary switch and check the GCS's messages tab for the latitude, longitude and alt (above sea-level)
+
+# How It Works
+
+The script's algorithm is implemented as follows
+
+1. Get the POI_DIST_MAX and TERRAIN_SPACING parameter values
+2. Get the vehicle Location (lat, lon, height above sea-level), initialise test-loc and prev-test-loc
+3. Get the vehicle's current alt-above-terrain
+4. Get gimbal attitude (only pitch and yaw are used)
+5. The test_loc variable is initialised to the vehicle's current location
+6. The prev_test_loc variable is a backup of test_loc
+7. test_loc is moved along the line defined by the gimbal's pitch and yaw by TERRAIN_SPACING (meters)
+8. Get the terrain's altitude (above sea-level) at test_loc
+9. Steps 6, 7 and 8 are repeated until test_loc's altitude falls below the terrain altitude
+10. Interpolate between test_loc and prev_test_loc to find the lat, lon, alt (above sea-level) where alt-above-terrain is zero
+11. Display the POI to the user