Prevent error when read_spectra sees unrecognized FITS extensions

[rongpu]

This PR allows desispec.io.read_spectra to read FITS files that contains unrecognized FITS extensions. Instead of throwing a fatal error, it now prints a warning and ignores the unrecognized extensions.

Example:

from desispec.io import read_spectra
spec = read_spectra('/pscratch/sd/r/rongpu/desi_dust_y1_release/spectra/main/bright/0/0/rvspecfit-main-bright-0.fits')

output:

WARNING:spectra.py:385:read_spectra: FITS extension REDSHIFTS is not recognized and is ignored
WARNING:spectra.py:385:read_spectra: FITS extension RVTAB is not recognized and is ignored
WARNING:spectra.py:385:read_spectra: FITS extension WAVELENGTH is not recognized and is ignored
WARNING:spectra.py:385:read_spectra: FITS extension SPECMOD is not recognized and is ignored
INFO:spectra.py:392:read_spectra: iotime 0.050 sec to read rvspecfit-main-bright-0.fits at 2025-02-06T13:07:39.662369

[coveralls]

coverage: 39.357%. remained the same
when pulling 415cf9c on readspectra
into d7eb389 on main.

[sbailey]

Looks good, thanks. I also verified that it still works on regular spectra and coadd files as expected.