add multipleBonds param to bond visuals

CHANGELOG.md

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

src/mol-repr/structure/visual/bond-inter-unit-cylinder.ts

src/mol-repr/structure/visual/bond-inter-unit-line.ts

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

src/mol-repr/structure/visual/bond-intra-unit-line.ts

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

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