basic mapped param support

src/mol-plugin/context.ts

src/mol-plugin/spec.ts

src/mol-plugin/state/actions/basic.ts

src/mol-plugin/state/transforms/visuals.ts

src/mol-plugin/ui/controls/parameters.tsx

src/mol-repr/representation.ts

src/mol-repr/shape/representation.ts

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

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

src/mol-repr/structure/units-representation.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