Fix/installation and eval

[belerico]

Summary

This PR updates the README to include installation steps also from the PyPi index, fixes the creation of the evaluation directory, which will be saved always inside the checkpoint directory (as specified in the how-to eval_your_agent.md page) , and creates three new console scripts so that the users can safely run evaluation, registration and look at the available agents from everywhere in the filesystem if they have installed SheepRL from PyPi.

Type of Change

Please select the one relevant option below:

• New feature (non-breaking change that adds functionality)
• Documentation update

Checklist

Please confirm that the following tasks have been completed:

• I have tested my changes locally and they work as expected. (Please describe the tests you performed.)
• I have added unit tests for my changes, or updated existing tests if necessary.
• I have updated the documentation, if applicable.
• I have installed pre-commit and run locally for my code changes.

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