Tidy up public interface

[alejandrosame]

Description

With this change the PyDP offers a tidy interface to the user. Now pydp exposes the submodules algorithms, distributions and util (and modules/classes under them) without polluting the namespaces with implementation details.

Affected Dependencies

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How has this been tested?

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Checklist

• [ ] I have followed the Contribution Guidelines and Code of Conduct
• [ ] I have commented my code following the OpenMined Styleguide
• [ ] I have labeled this PR with the relevant Type labels
• [ ] My changes are covered by tests