refactored principal-axes calculation

src/mol-math/linear-algebra/matrix/principal-axes.ts

src/mol-model/structure/structure/carbohydrates/compute.ts

src/mol-model/structure/structure/element/loci.ts

src/mol-model/structure/util.ts

src/mol-plugin/ui/structure/selection.tsx

src/mol-plugin/util/structure-element-selection.ts

src/mol-plugin/util/structure-orientation.ts