Yenta

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* Serialization scheme moved from JSON to pickle to improve performance.
* "Values" no longer explicitly exist; anything that can be pickled is a valid value.
* Multiple pipelines with different names can be used in the same project.

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* FileArtifacts can be directories
* Artifacts now have a `meta` field to which arbitrary JSON-serializable information can be attached
* If an exception is raised during task execution, the stack trace is printed
* pydot is now used for graph generation
* Artifact equality is checked by comparing location and hash

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* Initialize artifact date_created by default

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* Append running dir to sys.path

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* Write intermediate pipeline results to a temp file and catch the case
where a non-serializable object is returned

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* Selector functionality implemented