Artifact checks out to default dir if no --tar or --directory

Description

If neither --tar nor --directory are provided, bst artifact checkout will no longer error out, but default to a suitable directory. This is more consistent with bst workspace open.

Changes proposed in this merge request:

• Get rid of error message stating one of --tar or --directory must be provided
• Default to a directory when neither are provided
• Add new test for this feature

This merge request, when approved, will close: #1079 (closed)

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