Rui

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* Added example to README
* Works decently well now!

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* You can now remove entities from groups

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* Groups now return empty list if there is no group of that name

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* Updated usage documentation to fix typo
* Added partial (subset) component selection

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* Updated README with GitHub and PyPI links
* Systems can now be removed
* Entities can now be removed
* Entities will now check if they are alive or else raise an exception

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* Added API documentation