Npstreams

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* Added a changelog.
* Added the possibility to use weights in ``ihistogram``.
* Added the function ``average_and_var`` to compute the average and variance in a single pass.
* Documentation regarding the ``ddof`` keyword in many statistical wrongly stated that the default value was 1. This has been corrected.

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* Fixed some issues with NumPy versions above 1.16.

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* Added benchmarking capabilities.
* Added the ``array_stream`` decorator.
* Removed support for Python < 3.6.