Fix index retention in _tensor_truth function #157

Job	Run time
test (bench/rna.ipynb)	2m 52s
test (bulk/rna.ipynb)	1m 53s
test (scell/rna_psbk.ipynb)	1m 45s
test (omnipath/licenses.ipynb)	1m 2s
test (omnipath/orthologs.ipynb)	1m 27s
test (scell/rna_pstime.ipynb)	2m 0s
test (spatial/rna_visium.ipynb)	1m 43s
test (example.ipynb)	2m 4s
test (scell/rna_sc.ipynb)	1m 18s
	16m 4s

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