Upgrade or degrade resolution of a pixel list.

    Parameters:
    -----------
    ipix:array-like 
    the input pixel(s)

    nside_in:int
    the nside of the input pixel(s)

    nside_out:int
    the desired nside of the output pixel(s)

    order:str
    pixel ordering of input and output ("RING" or "NESTED")

    Returns:
    --------
    pix_out:array-like
    the upgraded or degraded pixel array

def ud_grade_ipix(ipix, nside_in, nside_out, nest=False):
    """
    Upgrade or degrade resolution of a pixel list.

    Parameters:
    -----------
    ipix:array-like 
    the input pixel(s)

    nside_in:int
    the nside of the input pixel(s)

    nside_out:int
    the desired nside of the output pixel(s)

    order:str
    pixel ordering of input and output ("RING" or "NESTED")

    Returns:
    --------
    pix_out:array-like
    the upgraded or degraded pixel array
    """
    if nside_in == nside_out: return ipix
    elif nside_in < nside_out:
        return u_grade_ipix(ipix, nside_in, nside_out, nest)
    elif nside_in > nside_out:
        return d_grade_ipix(ipix, nside_in, nside_out, nest)