Eigen can use the raw data of a vector directly by mapping the vector's internal array. This
should save a lot of copy calls. This is also the prefered way to "cast" a std::vector to an eigen vector. 

In this case we use it for "casting". 
In setDesiredErrorValues we use it to avoid copy calls.

http://eigen.tuxfamily.org/dox/group__TutorialMapClass.html