Recover cell barcode if it contains a single N only

[haghshenas]

In Salmon, there is a step for recovering barcodes that contain N's. If there is only one N, it will be replaced with A, otherwise that barcode will be ignored:
/~https://github.com/COMBINE-lab/salmon/blob/447ae4eef9bcf2b6293a01a99fbbd8d5007e370d/src/SalmonAlevin.cpp#L686-L694

It would be nice to implement the same logic here too.

[haghshenas]

Should this be done for UMI too? It seems Salmon only does this for barcode and not UMI.

[rob-p]

Good point — I think it makes sense to do it for UMIs as well. That should probably be the behavior we adopt during mapping within salmon itself too.