`BarrierOp` and `Circuit._add_conditional_barrier` #4158

Job	Run time
Check tket library version	17s
Check documentation build	1m 6s
Check C++ code formatting	2m 35s
Build and test (tket) (ubuntu-22.04)	5m 2s
Build and test (tket) (macos-12)	5m 5s
Build and test (tket) (windows-2022)	4m 45s
Build and test pytket (ubuntu)	5m 14s
Build and test tket (macos-arm64)	3m 52s
Build and test pytket (macos)	4m 57s
Build and test (tket)	0s
Build and test (tket)	0s
Build and test pytket (macos-arm64)	1m 14s
Build and test pytket (windows)	4m 48s
Check pytket line and branch coverage	1s
Publish pytket coverage	1s
	38m 57s

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