initial direct volume rendering version

src/apps/canvas/structure-view.ts

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

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

src/mol-geo/representation/structure/index.ts

...resentation/structure/representation/molecular-surface.ts

src/mol-geo/representation/structure/units-visual.ts

...epresentation/structure/visual/gaussian-density-volume.ts

src/mol-geo/representation/structure/visual/util/common.ts

src/mol-gl/render-object.ts

src/mol-gl/renderable/direct-volume.ts

src/mol-gl/renderer.ts

src/mol-gl/shader-code.ts

src/mol-gl/shader/direct-volume.frag

src/mol-gl/shader/direct-volume.vert

src/mol-math/geometry/common.ts

src/mol-math/interpolate.ts