Expand comparison plotting to 2+ endpoint samples per plot

[madison-feshuk]

Currently comparison plotting is limited to dual plots (i.e. where ordered m4ids supplied and the length of compare.val and val must be equal)

A few users have developed their own standalone plotting to plot 2+ endpoint samples per plot, but it would be nice to introduce this capability to tcpl. Some examples:

• ATG mixtures (3)
  
• Deisenroth DEVTOX (4)