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* core
* add option for input drivers to let processing-heavy tasks be handled by ``mapchete.Executor`` by implementing ``InputData.add_preprocessing_task()`` and ``InputData.get_preprocessing_task_result()`` methods
* check futures submitted to ``mapchete.Executor`` for exceptions before yielding
* remove deprecated ``multi`` (now called ``workers``), ``distributed`` and ``max_chunksize`` arguments
* CLI
* remove deprecated ``--max-chunksize`` option
* replace "tiles" with "tasks" in progress
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