Commit 2020-10-29 19:28 ccc98d0a

refactor(*): midpoint, point_reflection, and Mazur-Ulam in affine spaces (#4752)

• redefine midpoint for points in an affine space;
• redefine point_reflection for affine spaces (as equiv, affine_equiv, and isometric);
• define const_vsub as equiv, affine_equiv, and isometric;
• define affine_map.of_map_midpoint;
• fully migrate the proof of Mazur-Ulam theorem to affine spaces;