it allows the possiblity to save some compilation time by linking to it *and* defining the token EIGEN_EXTERN_INSTANCIATIONS
-fix CMakeLists, public headers weren't getting installed
1) Eigen2 co-installable with Eigen1 without conflict, without affecting programs including either. 2) #include<Eigen/Core> without the .h without conflict with the Core/ directory 3) Uniformize coding style of the CMakeLists.
in the examples instead of ugly things like #include"../../src/Core.h"
