Add a `new_observer_frame` for 2D transformations too.

[sebcrozet]

This addition is motivated by https://discourse.nphysics.org/t/trouble-determining-angle-between-vectors-points/281/5. This feature exists in 3D but not in 2D.

[wilmerdooley]

I opened #1605 to address this. It adds a face_towards constructor for 2D isometries (Isometry2 and IsometryMatrix2) mirroring the existing 3D behavior. The details are in the PR; happy to adjust based on what you'd prefer.