Add SubproblemCount attribute #362

Job	Run time
Julia 1 - ubuntu-latest - x64 - pull_request	17m 45s
Julia 1.10 - ubuntu-latest - x86 - pull_request	15m 2s
Julia 1.10 - ubuntu-latest - x64 - pull_request	14m 59s
Julia 1 - windows-latest - x64 - pull_request	19m 51s
	1h 7m 37s

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