Add Figure S4 panels and compiled versions

[sjspielman]

This PR adds Figure S4.

• Panels were apparently not yet in figures/pdfs/supp/figs4/panels, so panels were added there by:
  • Adding a step in generate_figures.sh to run the mutational-signatures module. This step was previously missing, and was added above the first line in this file that is copying a figure from that module
  • Adding lines to copy the two panels into figures/pdfs/fig4/panels
• A compiled PDF was added as figures/pdfs/supp/figs4/figureS4.pdf
• A compiled PNG was added as figures/pngs/figureS4.png

[sjspielman]

Code changes do not depend on CI so we can merge before checks are complete.