Add a singular s-matrix processing logic in Circuit.s_external and co… #305

Job	Run time
Tests (3.8)	1m 55s
Tests (3.9)	1m 58s
Tests (3.10)	1m 54s
Tests (3.11)	1m 41s
Tests (3.12)	1m 41s
Tests (3.13)	1m 55s
Indicate completion to coveralls.io (Finish)	12s
Publish Tests Results	8s
	11m 24s

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