Ensure imported barcodes have no zero counts

In https://github.com/AlexsLemonade/scpcaTools/pull/286 we updated the alevin-fry import to round counts when requested, but in doing so we can sometimes create SCE objects where there is a barcode with all zero counts. We should filter these out to avoid some potential downstream issues.

Start by reviewing pull request #286 and the alevin-fry import path it changed. Update the import behavior so resulting SCE objects exclude barcodes with all zero counts, then verify imported objects contain no all-zero barcodes.

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