wip, mc group id

src/mol-geo/geometry/isosurface/isosurface.ts

src/mol-gl/compute/marching-cubes/isosurface.ts

src/mol-gl/renderable/isosurface.ts

src/mol-gl/shader/chunks/assign-group.glsl

src/mol-gl/shader/chunks/assign-position.glsl

src/mol-gl/shader/marching-cubes/isosurface.frag

src/mol-gl/shader/mesh.vert

src/tests/browser/marching-cubes.ts