Adds a parallel manager, allowing parallel execution of reaktoro blocks. #104

Job	Run time
Check code is formatted (Black)	6s
pytest (linux/3.9/dev)	3m 21s
pytest (win64/3.9/dev)	16m 2s
pytest (linux/3.10/dev)	3m 28s
pytest (win64/3.10/dev)	13m 30s
pytest (linux/3.11/dev)	3m 44s
pytest (win64/3.11/dev)	14m 23s
pytest (linux/3.12/dev)	3m 27s
pytest (win64/3.12/dev)	14m 9s
	1h 12m 10s

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