Potential bug in _reindex

[Eikor]

Dear researchers,

Thanks for your amazing work!

I noticed a small potential bug in spectrum_datasets.py. I believe This line of code should be

for lin_idx in range(self._file_offsets[-1]):
    grp_idx += lin_idx >= self._file_offsets[grp_idx + 1]
    row_idx = lin_idx - self._file_offsets[grp_idx]
    self._locs[lin_idx] = (grp_idx-1, row_idx)

, the current version can cause an index error (len(dataset._locs) < len(dataset)) when reusing a preprocessed index file.

Thank you for your attention!

Best,
Junde