Aioraven

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* Handle filler value for DateCode

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* Add tests for timeout recovery
* Fix typing on RAVEnWriter constructor
* Expose mechanism to abort pending writes
* Add timeouts for stream device synchronization

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* Switch to pyserial-asyncio-fast to fix event loop being blocked

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* Fix rounding error for values just under a whole number

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* Defer feeding XML data until we likely trigger an event

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* Treat summation values as a signed number