209 include new generative plugin great

[robsdavis]

Description

This PR adds a new Plugin, GReaT. It is a lightweight wrapper around the library be_great.
This library has dependency conflicts with arfpy and so, this PR moves the arf plugin to an extra and implements this new plugin as an extra. These extras are not included in the all extra, so that the user can install with:

pip install .[all]
pip install .[arf]

or

pip install .[all]
pip install .[great]

depending on preference for one plugin or the other.

Affected Dependencies

Additional dependency: be_great.

How has this been tested?

• tests for the plugin are in "synthcity/tests/plugins/generic/test_great.py" and are analogous to other plugins such as goggle
• A tutorial is provided in "/home/rob/Documents/projects/synthcity/tutorials/plugins/generic/plugin_great.ipynb", which is tested by the nb_eval.py test when the all_tests option is applied.

Checklist

• I have followed the Contribution Guidelines and Code of Conduct
• I have commented my code following the van der Schaar Lab Styleguide
• I have labelled this PR with the relevant Type labels
• My changes are covered by tests