volume param tweaks, handle 0 sigma

src/mol-geo/geometry/direct-volume/direct-volume.ts

src/mol-model/volume/data.ts

src/mol-plugin/ui/controls/parameters.tsx

src/mol-repr/structure/visual/gaussian-density-volume.ts

src/mol-repr/volume/direct-volume.ts

src/mol-repr/volume/isosurface.ts