Rotation matrix handedness

[hayashi-stl]

Looking at the documentation for Basis2, it claims to be a rotation matrix. However, the guarantee is that it's orthogonal, which is not enough for a rotation matrix. The determinant has to be 1. In fact, Basis2::look_at_stable can create a reflection matrix. Should reflection matrices be allowed in Basis2 or should the hole in Basis2::look_at_stable be fixed and the documentation updated to say that Basis2 is guaranteed to be orthogonal and have a determinant of 1?

[kvark]

Did you see #508 that is deprecating look_at?

[hayashi-stl]

Okay, I'll wait and see if the replacement can still produce reflection matrices.