MLFlow compatibility

[Sinclert]

This PR adds support for the recent compatibility between MLFlow and the ML sub-workflow.

In order to achieve that:

• The USERNAME env. variable need to be specified, as REANA cannot find its current user on the /etc/passwd file.
• The MLFLOW_TRACKING_URI env. variable need to be specified, so the workflow knows where to store data.
• The reana/workflow.yml need to propagate MLFlow arguments downfall.

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Additional changes:

• GIT submodule references has been updated to the latest version of the Physics and ML sub-workflows.
• Documentation has been updated, including a new MLFlow section.

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This workflow is now REANA runnable (given a properly configured MLFlow tracking server) 🚀