Code Refactoring

[fealho]

Description

SDGym requires some refactoring. In part due to issue #212, parts of the code are currently misplaced, unused or outdated.

That should be fixed in the following parts:

• Create a _benchmark method which all benchmark_{modality} methods will call (add helper methods)
• Delete the MODALITIES attribute
• Delete SingleTableBaselineSynthesizer
• Fix MultiSingleTableBaselineSynthesizer
• Move all outdated code into a Legacy folder
• Move all CLI-only code into a CLI folder
• (Maybe) Convert all metadata objects into SingleTableMetadata/MultiTableMetadata
• dataset.py logic should import from SDV
• Remove SynthesizerMapping and rename SingleTablePreset to FastMLPreset
• Add integration tests for all public methods
• Review and update any incorrect docstrings/READMEs
• Other minor fixes

These should all be merged into a separate branch, which will in turn be merged to master.