Fix bug when reading transmision files with hdu named F_METALS.

[alxogm]

This PR fixes a bug found when trying to run quickquasar on the London mocks v8.0, and we are also using it to save global seed and pixel seed in the truth files. Output is as indicated below...

test=fits.open("/project/projectdirs/desi/users/alxogm/desi/mocks/london/metals/v8.0.0/truth-16-0.fits")
test.info()
Filename: /project/projectdirs/desi/users/alxogm/desi/mocks/london/metals/v8.0.0/truth-16-0.fits
No.    Name      Ver    Type      Cards   Dimensions   Format
  0  PRIMARY       1 PrimaryHDU       6   ()      
  1  TRUTH         1 BinTableHDU     51   5R x 18C   [K, 10A, 10A, I, K, E, E, 15A, E, E, E, E, E, E, D, D, D, E]   
  2  QSO_META      1 BinTableHDU     21   5R x 5C   [K, E, 5E, 186E, I]   

test[0].header
[32]:
SIMPLE  =                    T / conforms to FITS standard                      
BITPIX  =                    8 / array data type                                
NAXIS   =                    0 / number of array dimensions                     
EXTEND  =                    T                                                  
GSEED   =                  123                                                  
PIXSEED =                 2866   

[londumas]

Thanks @alxogm, I have a small request for the error message.

[alxogm]

OK. I'll merge now.