Restore quaternion group action

Commit 370631f01f71db36b580e5a65790b2a8d9559821 rescaled the quaternion
manifold's group action so the tangent vector norm matches the full
rotation angle, addressing issue #941. While both conventions are
mathematically valid, issue #311 already concluded that the half-angle
one should be kept because too much code depends on it. The rescaling
effectively broke that expectation for existing callers.

Restore the pre-370631f01f71db36b580e5a65790b2a8d9559821 scaling, keep
the rest of that commit's cleanup, and update the documentation and
tests to match.

Fixes #1190

Change-Id: I1dd189c51153ed62cc8c871472aacb87673ec351