Generalize some variables for plotting/analysis

[vtlim]

• It might better to have an argument for draw_density2d() in compare_ffs.py to generalize the x, y, z, ranges.

• color_by_moiety.py could be generalized to support more moiety groups. The script currently has three encoded color codes (see subset_colors) which was chosen for the manuscript figures to best distinguish between the three groups presented.