fix a bug in assert_grid_correct_after_powerflow #471

Job	Run time
Build macos-14 cp38 wheel with cibuildwheel	4m 38s
Build linux cp37 wheel on manylinux2014_x86_64	3m 22s
Build macos-14 cp39 wheel with cibuildwheel	1m 47s
Build windows cp37 wheel for x64	5m 2s
Build linux cp38 wheel on manylinux2014_x86_64	3m 21s
Build windows-2019 cp39 wheel with cibuildwheel	4m 20s
Build darwin cp37 wheel on x86_64	4m 17s
Build windows cp38 wheel for x64	5m 4s
Build linux cp39 wheel on manylinux2014_x86_64	3m 27s
Build windows-2019 cp310 wheel with cibuildwheel	5m 52s
Build darwin cp38 wheel on x86_64	3m 48s
Build windows cp39 wheel for x64	4m 47s
Build linux cp310 wheel on manylinux2014_x86_64	3m 20s
Build windows-2019 cp311 wheel with cibuildwheel	4m 29s
Build darwin cp39 wheel on x86_64	4m 47s
Build windows cp310 wheel for x64	7m 18s
Build linux cp311 wheel on manylinux2014_x86_64	3m 22s
Build windows-2019 cp312 wheel with cibuildwheel	4m 31s
Build darwin cp310 wheel on x86_64	4m 12s
Build windows cp311 wheel for x64	5m 7s
Build linux cp312 wheel on manylinux2014_x86_64	3m 28s
Build darwin cp310 wheel on arm64	3m 0s
Build windows cp312 wheel for x64	5m 34s
Build darwin cp311 wheel on x86_64	5m 50s
Build darwin cp311 wheel on arm64	2m 50s
Build darwin cp312 wheel on x86_64	5m 59s
Build darwin cp312 wheel on arm64	2m 50s
Package wheels	10s
	1h 56m 32s

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