Given coords and a site, find closet site to coords.
        Args:
            coords (3x1 array): cartesian coords of center of sphere
            site: site to find closest to coords
            r: radius of sphere. Defaults to diagonal of unit cell

        Returns:
            Closest site and distance.

def get_nearest_site(self, coords, site, r=None):
        """
        Given coords and a site, find closet site to coords.
        Args:
            coords (3x1 array): cartesian coords of center of sphere
            site: site to find closest to coords
            r: radius of sphere. Defaults to diagonal of unit cell

        Returns:
            Closest site and distance.
        """
        index = self.index(site)
        if r is None:
            r = np.linalg.norm(np.sum(self.lattice.matrix, axis=0))
        ns = self.get_sites_in_sphere(coords, r, include_index=True)
        # Get sites with identical index to site
        ns = [n for n in ns if n[2] == index]
        # Sort by distance to coords
        ns.sort(key=lambda x: x[1])
        # Return PeriodicSite and distance of closest image
        return ns[0][0:2]