Simex-lite

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* Refactor based on `libpyvinyl <https://github.com/PaNOSC-ViNYL/libpyvinyl>`_.
* Support `instrument` class.
* Updated documents for backengines installation.

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* Support for the legacy `DiffractionData` class.

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* Fixed reading singfel rewrited data.
* Add chunk functions to `GaussianNoiseCalculator`.

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* Included utils as a subpackage.

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* Updated the interface of data APIs. Details at this `PR link <https://github.com/PaNOSC-ViNYL/SimEx-Lite/pull/4>`_.
* Updated the deployment guide for PyPI in `CONTRIBUTING <https://github.com/PaNOSC-ViNYL/SimEx-Lite/blob/main/CONTRIBUTING.rst>`_.

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* Deploy PyPI with Travis