Make metadata more accessible in cell ratioplots

[aislyn]

Obviously you can make lists of cell names and put them into ratio plots, but I think you pretty much always want to look at an example cell from each subclone/sample to make sure the segmentation is accurate for that subgroup. I think it would save a lot of repetitive user code if plotRatioPlot() would easily separate cells by a specified metadata column.

[darlanminussi]

This could be interesting but I wonder if it should be its own function. Seems that a visualization as such on plotRatio could be confusing. plotConsensusLine tries to somewhat address that but yeah it is not really at the single-cell level. Could be an extra parameter in the plotRatio app. Leaving it open for now.