message: Remove Eigen compilation error for Alpha complex 3d
diff: # error Alpha_complex_3d is only available for CGAL >= 4.11
  #endif
  -#if !EIGEN_VERSION_AT_LEAST(3,1,0)
  -# error Alpha_complex_3d is only available for Eigen3 >= 3.1.0 installed with CGAL
  -#endif
  -
  namespace Gudhi {
  namespace alpha_complex {
  