Separate cell barcode counting and filtering

[olgabot]

Is your feature request related to a problem? Please describe

Currently, if --tenxt_min_umi_per_cell is changed, then count_umi_per_cell restarts from zero, even though all the UMIs are already counted, it's the filtering that happens separately. The count_umi_per_cell process takes ~5-10 hours to complete per bam file, so it would be ideal to not have to rerun it.

Describe the solution you'd like

Set the min umi per cell in count_umi_per_cell to 0, then separately use Nextflow to filter the per-barcode UMI counts using the params.tenx_min_umi_per_cell value.

Describe alternatives you've considered

Additional context