Change approach for allowing `seg_name` and `seg_len` to be lists

In https://github.com/MIT-LCP/wfdb-python/pull/514 an update was made to allow seg_name and seg_len to be lists. This is needed for writing a MultiSegmentRecord.

@bemoody , suggested an alternate approach:

But I think what we really want is, rather than changing the types in SEGMENT_SPECS, we want to add "seg_name" and "seg_len" to LIST_FIELDS (wfdb/io/record.py, line 1675.)

This issue is being opened to track implementing this alternate approach.

まず、wfdb/io/record.py の 1675 行付近にある LIST_FIELDS と、現在の SEGMENT_SPECS の処理を確認してください。seg_name と seg_len が LIST_FIELDS を通じて処理され、MultiSegmentRecord の書き込みが引き続きそれらをサポートすれば、変更は完了です。