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

[briangow]

In #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.