Add the function to get topological generations

[inmzhang]

Closes #1014

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[coveralls]

Pull Request Test Coverage Report for Build 7522341357

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.03%) to 95.923%

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Totals
Change from base Build 7513108802:	-0.03%
Covered Lines:	16024
Relevant Lines:	16705

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💛 - Coveralls

[IvanIsCoding]

LGTM, thanks for making your first contribution!

Overall the code looks good and I don't think there is a need to change anything for functionality. I left one optional comment about the DAG check.

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