Adds a parallel manager, allowing parallel execution of reaktoro blocks. #93

Job	Run time
Check code is formatted (Black)	10s
pytest (linux/3.9/dev)	3m 12s
pytest (win64/3.9/dev)	12m 7s
pytest (linux/3.10/dev)	3m 43s
pytest (win64/3.10/dev)	13m 0s
pytest (linux/3.11/dev)	4m 7s
pytest (win64/3.11/dev)	15m 20s
pytest (linux/3.12/dev)	3m 16s
pytest (win64/3.12/dev)	14m 22s
	1h 9m 17s

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