Extremitypathfinder

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IMPORTANT BUGFIX: in some cases the visibility computation was faulty (fix 23)

* added new test case

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* IMPROVEMENT: Different polygons may now intersect each other. Thanks to `Georg Hess <https://github.com/georghess>`__!
* BUGFIX: Fixed a bug that caused "dangling" extremities in the graph to be left out
* ``TypeError`` and ``ValueError`` are being raised instead of ``AssertionError`` in case of invalid input parameters with ``validate=True``. Thanks to `Andrew Costello <https://github.com/Andrewsyl>`__!

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* BUGFIX: fix 16. introduce unique ordering of A* search heap queue with custom class ``SearchState`` (internal)

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* BUGFIX: fix clockwise polygon numbering test (for input data validation, mentioned in 12)

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* FIX 11: added option ``verify`` to ``find_shortest_path()`` for skipping the 'within map' test for goal and start points

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* supporting only python 3.7+
* fix 10: Memory leak in DirectedHeuristicGraph
* fix BUG where "dangling" extremities in the visibility graph would be deleted
* using generators to refer to the polygon properties (vertices,...) of an environment (save memory and remove redundancy)
* enabled plotting the test results, at the same time this is testing the plotting functionality
* added typing

internal:

* added sphinx documentation, included auto api documentation, improved docstrings
* added contribution guidelines
* add sponsor button
* updated publishing routine
* split up requirement files (basic, tests)
* specific tags for supported python versions in wheel
* testing all different python versions with tox
* added coverage tests
* added editorconfig
* specify version in VERSION file
* added new tests