FindEigen3.cmake has mixed EIGEN and EIGEN3 variables

A number of warnings regarding conflict between Eigen and Eigen3 naming in the cmake .. process I think is due to our cmake_modules/FindEigen3.cmake file being oddly coded.

In particular, our file mixes instances of EIGEN_... variables with EIGEN3_... variables.

Instead, the file in the eigen library is made exclusively with EIGEN3 entries:

See the original file here https://github.com/libigl/eigen/blob/master/cmake/FindEigen3.cmake

I think we should fix this.