RxSO(2) and Sim(2)

Hello Hauke, 

I have successfully merged your current develop branch. Just encountered one issue that the defined tan(*) function in ceres_eigen.hpp won't compile since the Ceres has defined them in jet.h (exactly the same). Is this probably because the Ceres version on my machine is different from yours? But anyway you can make the decision for wether to merge it.

Cheers,
Pinglin