Add SDSS DECam Standard Star Catalog

[Yong2Sheng]

Summary

1. Add the tutorial notebook for SDSS DECam Standard Star Catalog
2. Write the catalog source webpage address to the table attributes.
3. If ra_err and dec_err are missing in the original text file, the code will create these two missing columns with NaN values:

for chunk in pbar_chunks:
    n = len(chunk)

    if "ra_err" not in chunk.columns:
        chunk["ra_err"] = np.full(n, np.nan, dtype=np.float32)

    if "dec_err" not in chunk.columns:
        chunk["dec_err"] = np.full(n, np.nan, dtype=np.float32)

    yield file, chunk