Allow expanded parameter in MultiRecord.wrsamp

[briangow]

As raised in #554, wfdb.io.Multirecord.wrsamp does not provide the option to pass expanded=True. This should be provided so that multifrequency signals can be written within the Multirecord class. This PR updates the wrsamp call to allow for expanded to be passed.

[bemoody]

This makes sense. But I think we should probably also have more validity checking when writing out multisegment records.

_check_segment_cohesion looks like somebody started to do that, but that function isn't used?!

[briangow]

Thanks @bemoody! I agree with your comment about additional validation. I've opened issue #557 to capture that. We can use separate PRs to implement those validations.