code: def get_sites_in_sphere(self, pt, r):
          
          neighbors = []
          for site in self._sites:
              dist = site.distance_from_point(pt)
              if dist <= r:
                  neighbors.append((site, dist))
          return neighbors
docs: Find all sites within a sphere from a point.
  
          Args:
              pt (3x1 array): Cartesian coordinates of center of sphere.
              r (float): Radius of sphere.
  
          Returns:
              [(site, dist) ...] since most of the time, subsequent processing
              requires the distance.