Allow transformations to set a warning flag to indicate that the pipeline has finished with warnings

[yruslan]

Background

A postprocessing step can lead to errors or warnings in the dataset.

Errors are easy - an exception can be thrown.

But warnings are not possible to add at the moment.

Feature

Allow transformations to set a warning flag to indicate that the pipeline has finished with warnings.

Example

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Proposed Solution [Optional]

Add a warning flag and give access to it from Pramen trait, e.g.:

Pramen.instance.setWarningFlag()