removed gaussian density calc from unit

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

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

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

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

src/mol-repr/structure/visual/gaussian-surface-mesh.ts

src/mol-repr/structure/visual/gaussian-surface-wireframe.ts

src/mol-repr/structure/visual/util/gaussian.ts