calculate gausian density as unit property

src/apps/canvas/component/structure-representation.tsx

...ol-geo/representation/structure/representation/surface.ts

...representation/structure/visual/gaussian-density-point.ts

.../representation/structure/visual/gaussian-surface-mesh.ts

...esentation/structure/visual/gaussian-surface-wireframe.ts

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

src/mol-math/geometry/common.ts

src/mol-math/geometry/gaussian-density.ts

src/mol-model/structure/structure/unit.ts

src/mol-model/structure/structure/unit/gaussian-density.ts