CellRanger Multi => add combined h5ad files in mtx_conversions

[nick-youngblut]

Description of feature

At least for v2.7.1, when running:

nextflow run main.nf \
  --aligner cellrangermulti \
  --skip_cellrangermulti_vdjref \
  --skip_emptydrops \
  --gex_frna_probe_set Chromium_Human_Transcriptome_Probe_Set_v1.0.1_GRCh38-2020-A.csv \
  --cellranger_multi_barcodes sample_barcodes.csv \
  --cellranger_index refdata-gex-GRCh38-2020-A/ \
  --input samples.csv \
  --outdir scrnaseq_output

The output mtx_conversions directory does not contain the combined_filtered_matrix.h5ad and combined_raw_matrix.h5ad matrix files generated when running the standard --aligner cellranger workflow. It would be helpful to have these combined matrix files.

[grst]

Depends on #310