Use the SampleSet key or the first Sample key in checkpoint wrapper

Uses a sample's key if key is not an attribute of the SampleSet.

Fixes #190 (closed).

assigned to @ydayer

changed the description

I need to add some tests to check this case.

mentioned in merge request !320 (merged)

added 4 commits

• 371ac802...666b29c7 - 3 commits from branch master
• 9b018359 - [py] SampleSet key or 1st Sample key in checkpoint

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merged

mentioned in commit 0a913967