Common superclass for transformers

[lars-reimann]

Is your feature request related to a problem?

Transformers currently contain a lot of copied & pasted documentation. We also don't have a type to express that we expect/return any transformer.

Desired solution

A common superclass (abstract base class) for transformers:

• TableTransformer (fit, transform, fit_transform)
• InvertibleTableTransformer (adds an inverse_transform method)

Possible alternatives (optional)

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Screenshots (optional)

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Additional Context (optional)

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[lars-reimann]

The fit methods of the different existing transformers currently don't have a common interface. Some can be applied to only one column, some can only be applied to all columns, some can be applied to a list of columns. This must be normalized first.