Timezonefinder

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BUGFIX: a longitude of 180 equals -180 (not 0.0 as previously implemented)

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* added new class TimezonefinderL for using JUST shortcuts (without timezone polygon data)
* therefore included the most common timezone of each shortcut stored in the binary file ``shortcuts_direct_id.bin``
* introduced typing
* included API documentation
* read hole registry directly from json, ``hole_poly_ids.bin`` not required any more
* added the ``parse_data.sh`` shell script for downloading the latest timezone data, also with oceans


improvements of file_converter.py:

* added command line arguments for specifying the input and output directories
* read binary names from ``global_settings.py``
* read data types from ``global_settings.py``
* use with statement for writing binaries
* automatically detect overflow for each data type in use
* cleanup code, remove redundancies, improve codestyle
* fixing 101: make imports work for local and remote execution

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* BUGFIX 99: include the correct timezone_names.json in build
* wheel specific for the supported python versions (3.6, 3.7, 3.8)

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* updated the data to `2020a <https://github.com/evansiroky/timezone-boundary-builder/releases/tag/2020a>`__
* added "extra" simplifying the installation of Numba
* added minimal required python version
* added minimal required version of the dependencies
* simplified and updated settings (e.g. reading current version from file)
* also testing python 3.8 now
* loading version from file

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* added option to specify the location of the binary data files to use. making it possible to easily point to own compiled data. also load timezone names json from this location
* make timezone names a class attribute (instead of a global variable)
* simplify code for opening and closing multiple binary files
* added tests for a specified path to the data
* testing multiple python3 versions automatically
* pinned new requirements
* importlib_resources removed from the dependencies
* added a documentation at: https://timezonefinder.readthedocs.io/en/latest/
* added contribution guidelines

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* updated the data to `2019b <https://github.com/evansiroky/timezone-boundary-builder/releases/tag/2019b>`__
* added description of using vectorized input in readme