Consider a separate directory for reference files

[sjspielman]

Currently in the file structure for scRNA-seq, we have some reference files (eg mitochondrial genes) that hang out in data/ directories, but these aren't exactly data! It would be a bit of a lift to overhaul this, but we might consider having a somewhat different structure for these files and placing them into a separate reference_files/ directory.

[sjspielman]

Worth noting that we are doing this for scRNA-seq-advanced for - data/reference/hs_mitochondrial_genes.tsv