Integrate Cell Ranger for reprocessing single-cell datasets based on 10x Chromium

[arteymix]

• detect platforms/datasets that are suitable for Cell Ranger
• add a task for generating the reference (optional, we can use those provided by Cell Ranger)
• add a task for running Cell Ranger, we might have to collect outputs in a new directory
• load single-cell data in Gemma with loadSingleCellData CLI using the --data-path option

[arteymix]

Those reference generating tasks should subclass luigi.ExternalTask if they do not provide any run() implementation.