vec3 improvements, .matchDirection & readonly .negUnit

src/mol-geo/geometry/mesh/builder/cylinder.ts

src/mol-math/linear-algebra/3d/mat4.ts

src/mol-math/linear-algebra/3d/vec3.ts

src/mol-repr/structure/visual/util/polymer/curve-segment.ts

src/mol-repr/structure/visual/util/polymer/trace-iterator.ts