Drop support for SuperLU 3.x

SuperLU 4.0 was released in 2009, SuperLU 3.0 in 2004. Since Jessie even Debian provides a newer packaged version of SuperLU then 3.0 (they provide 4.3).

I suggest to remove the support for older versions, as every version has changes and we try to detect them all in our CMake check. This would simplify the code and make it hopefully a little bit easier to add super for SuperLU 5.0 later on.

Please check your SuperLU version and the one you can get from your distribution. This is now the time to trigger / create an updated SuperLU package for your distribution.