Implement point cloud alignment for SE3

[Hs293Go]

Point cloud alignment is an application common enough that Eigen provides Eigen::umeyama to this end. Sophus can do better by exploiting a closed form solution for the optimum rotation quaternion. Another benefit is being able to store the result naturally in a SE3 object.

Method	Solver method	Result storage
Eigen::umeyama	SVD	Eigen::Matrix4d (16 parameters)
SE3::setFromPointClouds	Eigenproblem	Sophus::SE3 (7 parameters)

[coveralls]

Coverage increased (+0.03%) to 99.044% when pulling ecd252a on Hs293Go:develop into 0f7f957 on strasdat:master.