adds a method to LocationLocal that returns distance from home, in me… (#679)

* adds a method to LocationLocal that returns distance from home, in meters

* Return 3D distance to home if `down` is known, otherwise return 2D distance

* also, rename `distance` to `distance_home`

Author: wsilva32

dronekit/__init__.py

@@ -184,6 +184,17 @@ def __init__(self, north, east, down):
     def __str__(self):
         return "LocationLocal:north=%s,east=%s,down=%s" % (self.north, self.east, self.down)
 
+    def distance_home(self):
+        """
+        Distance away from home, in meters. Returns 3D distance if `down` is known, otherwise 2D distance.
+        """
+
+        if self.north is not None and self.east is not None:
+            if self.down is not None:
+                return math.sqrt(self.north**2 + self.east**2 + self.down**2) 
+            else:
+                return math.sqrt(self.north**2 + self.east**2)
+
 
 class GPSInfo(object):
     """