Add support for 10xGenomics fixed RNA profiling ("Flex")

[pjbriggs]

Adds support for 10x Genomics fixed RNA profiling ("Flex") data.

Details of the fixed RNA profiling can be found in the CellRanger documentation:

• What is Fixed RNA Profiling? https://support.10xgenomics.com/single-cell-gene-expression/software/pipelines/latest/what-is-frp
• Fixed RNA Profiling with cellranger multi https://support.10xgenomics.com/single-cell-gene-expression/software/pipelines/latest/using/multi-frp

The updates include a new QC protocol 10x_Flex which is automatically used when the single cell platform is 10xGenomics Chromium 3' and the library type is Flex.