Spooq

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* [ADD] Add Null Cleaner spooq.transformer.NullCleaner

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* [MOD] Store null value instead of an empty Map in case no cleansing was necessary

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* [MOD] Change logic for storing cleansed values as MapType Column to not break Spark (logical plan got to big)
* [MOD] Add streaming tests (parquet & delta) for EnumCleaner unit tests.

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* Left out intentionally as there is already a yanked version 3.3.2 on PyPi

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* [MOD] Add option to store logged cleansed values as MapType (Enum & Threshold based cleansers)
* [FIX] Fix TOC on PyPi, add more links to PyPi

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* [MOD] (BREAKING CHANGE!) rename package back to Spooq (dropping "2").
This means you need to update all imports from spooq2.xxx.yyy to spooq.xxx.yyy in your code!
* [MOD] Prepare for PyPi release
* [MOD] Drop official support for Spark 2.x (it most probably still works without issues,
but some tests fail on Spark2 due to different columns ordering and the effort is too high to
maintain both versions with respect to tests)