<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def load_file_or_hdu(filename):
 """ Load a file from disk and return an HDUList If filename is already an HDUList return that instead Parameters filename : str or HDUList File or HDU to be loaded Returns ------- hdulist : HDUList """

    if isinstance(filename, fits.HDUList):
        hdulist = filename
    else:
        hdulist = fits.open(filename, ignore_missing_end=True)
    return hdulist