Update IsNNN400: now 128 MCMC steps per sample instead of 32.

Fix missing bytes (due to github.com/apache/arrow/issues/41388)

Fix column name: 'samples' -> 'sample'

Organize files according to https://huggingface.co/docs/datasets/main/en/repository_structure

Change 'samples' column type from fixed_len_binary_array to binary_array because the former is not supported by HF.

IsNNN400 dataset