Pipeline Object

[LordDarkula]

A Pipeline object has been created with #710. Previously a pipeline could be generated with the to_pipeline method from a series of artifacts and dependencies. The issue with this approach is that since the output of this function is a set of python files, there was no way to regenerate the pipeline without calling to_pipeline again with the exact same set of arguments. Now we introduce an intermediate Pipeline object that stores the artifact and dependency relationships and uses an export method to generate the files for pipeline orchestration. Just like artifacts, pipelines can be saved with save and fetched in any session with get_pipeline.