CollectMultipleMetrics Skip making charts if R is not installed

[nh13]

I installed picard via picard-slim on bioconda, as I don't want to bloat my environment with an R install. I don't want/need the charts/histograms, so I was expecting the tool to work just fine, as the individual tools don't require that a chart be output. Thoughts on a PR to adding an option SKIP_PLOTS or whatever your preference would be?

[matthdsm]

This would be quite useful for ALL tools which use R or any other external dependency to draw plots.