overhaul regression fit/predict algorithms

[Andreas Janz]

Regression algorithms need to be overhauled.

This will also adress several issues: #487, #924, #960

• implement several PrepareRegressionDatasetFrom*Algorithm algorithms
• overhaul fit and predict algorithms
• implement a custom processing widget for on-the-fly dataset creation, similar to ProcessingParameterPickleFileClassificationDatasetWidget
• add Apply model > model.pkl > Predict regression layer option in the DataView context menu

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[Andreas Janz]

implemented fit base algorithm

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[Andreas Janz]

implemented predict regression algorithm

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[Andreas Janz]

implemented regressors

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[Andreas Janz]

implemented regression dataset creation algorithms

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[Andreas Janz]

implemented custom processing widget for on-the-fly dataset creation

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[Andreas Janz]

added Apply model context menu option

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[Andreas Janz]

resolves #487, #924, #960 and #959

→ <<cset 1a0005d09333>>