code: def get_neighbors_in_shell(self, origin, r, dr):
          
          outer = self.get_sites_in_sphere(origin, r + dr)
          inner = r - dr
          return [(site, dist) for (site, dist) in outer if dist > inner]
docs: Returns all sites in a shell centered on origin (coords) between radii
          r-dr and r+dr.
  
          Args:
              origin (3x1 array): Cartesian coordinates of center of sphere.
              r (float): Inner radius of shell.
              dr (float): Width of shell.
  
          Returns:
              [(site, dist) ...] since most of the time, subsequent processing
              requires the distance.