Use Euler parameters in QuaternionPlusImpl

Define the relation between the axis-angle representation of a rotation
and the corresponding unit quaternion as

  q(𝐞) = cos(𝜃/2) + sin(𝜃/2)/𝜃·𝐞

where 𝐞 = 𝜃𝛚 is the rotation vector given by the rotation angle 𝜃 and
the unit axis of rotation 𝛚 instead of

  q(𝐞) = cos(𝜃) + sin(𝜃)/𝜃·𝐞 .

This brings the former relation closer to the available functionality
provided by ceres/rotation.h.

Fixes #941

Change-Id: Id40c065f78593887ecc52e67a068737d7eb5bda6