Handle "reads per cell" in 10xGenomics metrics file being non-integer value when reporting multiplexed QC

[pjbriggs]

Handles the special case of the "reads per cell" value in a cellranger multi metrics_summary.csv file not being an integer when reporting 10x Genomics multiplexed statistics.

For example if no cells were assigned to a multiplexed sample then the metrics_summary.csv file might contain the line:

Cells,Gene Expression,,,Mean reads per cell,---

compared with:

Cells,Gene Expression,,,Mean reads per cell,"20,614"

Without the bugfix the QC reporting fails as the SampleQCReporter.get_10x_value() method in qc/reporting.py encounters an untrapped error attempting to format a non-integer value for the 10x_reads_per_cell field. The fix allows for non-integer values specifically for this field to be return as-is as string instead.