mol-plugin: focus camera on loci select behavior

src/mol-canvas3d/camera.ts

src/mol-math/geometry/centroid-helper.ts

src/mol-model/loci.ts

src/mol-model/structure/util.ts

src/mol-plugin/behavior.ts

src/mol-plugin/behavior/behavior.ts

src/mol-plugin/behavior/dynamic/camera.ts

src/mol-plugin/behavior/dynamic/representation.ts

src/mol-plugin/index.ts