Insert a site to the structure.

        Args:
            i (int): Index to insert site
            species (species-like): Species of inserted site
            coords (3x1 array): Coordinates of inserted site
            coords_are_cartesian (bool): Whether coordinates are cartesian.
                Defaults to False.
            validate_proximity (bool): Whether to check if inserted site is
                too close to an existing site. Defaults to False.
            properties (dict): Properties associated with the site.

        Returns:
            New structure with inserted site.

def insert(self, i, species, coords, coords_are_cartesian=False,
               validate_proximity=False, properties=None):
        """
        Insert a site to the structure.

        Args:
            i (int): Index to insert site
            species (species-like): Species of inserted site
            coords (3x1 array): Coordinates of inserted site
            coords_are_cartesian (bool): Whether coordinates are cartesian.
                Defaults to False.
            validate_proximity (bool): Whether to check if inserted site is
                too close to an existing site. Defaults to False.
            properties (dict): Properties associated with the site.

        Returns:
            New structure with inserted site.
        """
        if not coords_are_cartesian:
            new_site = PeriodicSite(species, coords, self._lattice,
                                    properties=properties)
        else:
            frac_coords = self._lattice.get_fractional_coords(coords)
            new_site = PeriodicSite(species, frac_coords, self._lattice,
                                    properties=properties)

        if validate_proximity:
            for site in self:
                if site.distance(new_site) < self.DISTANCE_TOLERANCE:
                    raise ValueError("New site is too close to an existing "
                                     "site!")

        self._sites.insert(i, new_site)