Add validation to MultiRecord.wrsamp for segment cohesion

[briangow]

As mentioned in #557, we should add segment validation when writing multi-segment records. This PR adds pre-write validation to MultiRecord.wrsamp() by activating the previously unused _check_segment_cohesion() method, which is now called before any files are written.

The validation includes checks for basic segment cohesion (matching n_seg, sampling frequencies, and segment lengths) as well as new validation for expanded signals. For segments with e_d_signal or e_p_signal, the method now verifies that samps_per_frame is properly set for all channels and that expanded signal lengths match expected values (samps_per_frame[ch] * sig_len). This provides clear, descriptive error messages when segments are misconfigured, preventing invalid multi-segment records from being written.