Pylt3

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* Added :code:`normalize_digits` to :code:`preprocessing`. This tiny function replaces all digits by ones in a file.

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* Changed the :code:`extension` parameter for :code:`concatenate_files` so that the dot has to be provided as well.
This allows you to basically look for filenames ending with any string rather than just a final extension.

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* Added the :code:`naive` tokenizer option when tokenizing in :code:`preprocessing.py`. This ensures that external
libraries need not be downloaded. The naive tokenizer does two things: replace word boundaries by a single
whitespace character, and substitute sequences of whitespaces with a single whitespace.

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* BUGFIX in :code:`print_simple_dict()` inside :code:`file_helpers`.

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* Added the :code:`keep_empty` option to :code:`tokenize_file` in :code:`preprocessing.py` so that empty lines are
still printed when required (default: :code:`True`).

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* Renamed :code:`tokenize` to :code:`tokenize_file` in :code:`preprocessing.py`.
* Added :code:`tokenize_string` in :code:`preprocessing.py`.

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