add scRNAseq processing function

[dtm2451]

Essentially the subset of the CITEseq 'subcluster' function which applies to the RNA assay, just with generation of umap on the uncorrected PCA added in.

Also:

• adds 'assays_keep' input to both functions (which defaults to the original assays hard-coded to be kept in the CITEseq version)

Question:

1. What should this function's directory be called? CITEseq version was called 'subcluster' but this almost feels like a misnomer since the functions can be used for the initial processing of a full merged dataset!