Adds a parallel manager, allowing parallel execution of reaktoro blocks. #94

Job	Run time
Check code is formatted (Black)	6s
pytest (linux/3.9/dev)	3m 25s
pytest (win64/3.9/dev)	13m 54s
pytest (linux/3.10/dev)	3m 31s
pytest (win64/3.10/dev)	15m 3s
pytest (linux/3.11/dev)	3m 49s
pytest (win64/3.11/dev)	11m 32s
pytest (linux/3.12/dev)	3m 15s
pytest (win64/3.12/dev)	14m 42s
	1h 9m 17s

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