Created struct HighsParallelSearch and instantiated a vector of them #2641

Job	Run time
debug (macos-latest)	7m 15s
debug_all_tests (macos-latest)	8m 14s
debug64 (macos-latest)	7m 45s
debug64_all_tests (macos-latest)	12m 43s
release (macos-latest)	10m 54s
Release64 (macos-latest)	12m 37s
release_all_tests (macos-latest)	5m 14s
Release64_all_tests (macos-latest)	6m 59s
	1h 11m 41s

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