sc_filterGenes: find genes that pass criteria
In pcahan1/singleCellNet: cell identity from single cell RNA-Seq data

based on idea that reliably detected genes will either be detected in many cells, or highly expressed in a small cels of cells (or both

1	sc_filterGenes(geneStats, alpha1 = 0.1, alpha2 = 0.01, mu = 2)

`geneStats`	result of running sc_statTab
`alpha1`	proportion of cells in which a gene must be considered detected (as defined in geneStats)
`alpha2`	lower proportion of cells for genes that must have higher expression level
`mu`	threshold, average expression level of genes passing the lower proportion criteria