Replace median for mean

Use the mean to compute the average distance of neighboring data points.

harmonica/_equivalent_sources/cartesian.py

@@ -264,7 +264,7 @@ def _build_points(self, coordinates):
         if self.block_size is not None:
             coordinates = self._block_average_coordinates(coordinates)
         if self.depth == "default":
-            self.depth_ = 4.5 * np.median(vd.median_distance(coordinates, k_nearest=1))
+            self.depth_ = 4.5 * np.mean(vd.median_distance(coordinates, k_nearest=1))
         else:
             self.depth_ = self.depth
         return (