volume repr refactoring

src/apps/structure-info/volume.ts

src/mol-repr/structure/complex-visual.ts

src/mol-repr/structure/index.ts

src/mol-repr/structure/units-visual.ts

src/mol-repr/structure/visual/carbohydrate-link-cylinder.ts

src/mol-repr/structure/visual/carbohydrate-symbol-mesh.ts

...ol-repr/structure/visual/cross-link-restraint-cylinder.ts

src/mol-repr/structure/visual/element-point.ts

src/mol-repr/structure/visual/element-sphere.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/inter-unit-link-cylinder.ts

src/mol-repr/structure/visual/intra-unit-link-cylinder.ts

src/mol-repr/structure/visual/polymer-backbone-cylinder.ts

src/mol-repr/structure/visual/polymer-gap-cylinder.ts

src/mol-repr/structure/visual/polymer-trace-mesh.ts

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

src/mol-repr/util.ts