Update how artifacts work
State: closed | Pull request: yes

This PR changes the following:

* how register_artifacts work
* how save_to works

The usage of both register_artifacts and save_to method remains identical.
Key differences in behaiviour:
* register_artifacts always guarantees to return existing path to artifact at the time of call. 
* save_to will replace and hash artifact names in config file before creating .nemo file. This is necessary to prevent name collisions
* when restoring artifacts from .nemo file it will create a temporary folder with artifacts for the lifetime of a model
* temporary folders associated with the model get deleted by destructor


TODOs:

* Some unittests fail
* Model parallel save_to (?)


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@titu1994 and @ericharper are we good to merge this?

Yes looks good to me. 

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