Method to sum two densities.

    Args:
        density1: First density.
        density2: Second density.

    Returns:
        Dict of {spin: density}.

def add_densities(density1, density2):
    """
    Method to sum two densities.

    Args:
        density1: First density.
        density2: Second density.

    Returns:
        Dict of {spin: density}.
    """
    return {spin: np.array(density1[spin]) + np.array(density2[spin])
            for spin in density1.keys()}