Add validation to MultiRecord.wrsamp for segment cohesion - #559

Thanks Brian, checking all the segments before writing anything is a good change. On main, a segment whose length doesn't match seg_len gets written without any error, and a bad segment partway through leaves half-written files. This PR fixes both.

I tried a few cases locally, and two writes that work on main are now rejected:

1. The expanded check runs even when expanded=False. A segment with a valid d_signal and an out-of-date e_d_signal now fails, even though only d_signal is written. The check also falls back to e_p_signal, which is never written. Could we pass expanded into _check_segment_cohesion and only check e_d_signal when it's True?
2. samps_per_frame=[None] is rejected. Record.wrsamp(expanded=True) treats a missing value as 1 (check_sig_cohesion in _signal.py), so a segment that writes fine on its own now fails inside a multi-segment record. It would be good to treat it the same way here.

Both of these come from having the expanded-length rule in two places. Another option is to drop the new block and run each segment's existing checks (its header field checks plus check_sig_cohesion) in a first pass, then write everything in a second pass. That keeps the "check before writing" behaviour with a single rule.

A few smaller things:

• Gap segments (None, e.g. from reading a variable-layout record with sampto) now fail with 'NoneType' object has no attribute 'fs'. That isn't worse than before, but a clear error saying gap segments can't be written yet would help.
• There are no tests for MultiRecord.wrsamp yet. It would be good to add:
  • a round trip of 041s
  • a seg_len mismatch
  • a bad segment after a good one, checking that no files are written
  • the two cases above that should be accepted
• Could you add a changelog entry to docs/changes.rst?
• A few of the new error message lines are over 88 characters.