Dataframe datamodule

[manuelkonrad]

📝 Description

• This PR adds the Dataframe datamodule which is instantiated directly from a pandas DataFrame. It is an alternative to the Folder datamodule for custom datasets where the labels are not encoded in the directory structure. Useful for situations where labels are refined regularly or for sub-sampling large datasets without copying or moving files.
• The datamodule also includes a from_file constructor which loads the data from a tabular file supported by pandas. The file format is given as argument but I could also add explicit constructors such as from_csv or from_parquet.
• As an alternative to Dataframe, I could rename the datamodule to Tabular in order to avoid confusion with pandas' DataFrame class.

✨ Changes

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• 🐞 Bug fix (non-breaking change which fixes an issue)
• 🔨 Refactor (non-breaking change which refactors the code base)
• 🚀 New feature (non-breaking change which adds functionality)
• 💥 Breaking change (fix or feature that would cause existing functionality to not work as expected)
• 📚 Documentation update
• 🔒 Security update

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• 📚 I have made the necessary updates to the documentation (if applicable).
• 🧪 I have written tests that support my changes and prove that my fix is effective or my feature works (if applicable).

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