ptGetTop: Find the best gene pairs for training
In pcahan1/singleCellNet: cell identity from single cell RNA-Seq data

Find the gene pairs that most distinguish a cancer group from the rest

1 2	ptGetTop(expDat, cell_labels, cgenes_list = NA, topX = 50, sliceSize = 5000, quickPairs = TRUE)

`expDat`	expDat
`cell_labels`	named vector, value is grp, name is cell name
`cgenes_list`	the list of labelled cgenes
`topX`	number of genepairs for training
`sliceSize`	the size of the slice for pair transform. Default at 5e3
`quickPairs`	TRUE if wanting to select the gene pairs in a quick fashion