Rename the sequence_length.txt file name

Summary

This is note from Michael email regarding the sequence_lengths.txt file: The sequence_lengths.txt file: Instead of containing the sequence length (20 in our 'standard' case = 10 context + 10 prediction frames), we store the number sequences per TFrecord-file therein. That's obviously confusing

What is the expected correct behavior?

Rename the sequence-lengths.txt file to number_sequences.txt