No more separate Expmap/Logmap derivatives...

[Frank Dellaert]

I'd like to see the ExpmapDerivative and LogmapDerivative disappear as separate functions. The key is to do it in such a way that the computation in Expmap and Logmap gets re-used optimally. This means understanding the earlier example optimizations, not just replace them.

@thduynguyen in coordination with @lucacarlone ?