Add "Volcano Plot" to main (is at EU and included in GTN tutorials)

[jennaj]

MTS: https://toolshed.g2.bx.psu.edu/view/iuc/volcanoplot/6d532d760950

cc @erasche @bgruening Looks like versions 0.0.2 + 0.0.3 are both exposed in the tool panel at EU. The versions menu for both finds all versions. Maybe on purpose but if not FYI :)