initial implementation of IncidenceGraphInterface.subgraph method #633

Job	Run time
lint/style-and-typos	57s
linux/3.8/bare-env	15s
linux/3.12	15m 9s
osx/3.10	24m 25s
win/3.9	43m 28s
linux/3.11/conda	18m 46s
linux/3.9/mpi	9m 25s
linux/3.10/cython	18m 29s
win/3.8/pip	25m 32s
process-coverage-${{ matrix.TARGET }}	0s
	2h 36m 26s

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