Restore CMake variables EIGEN3_INCLUDE_DIR, EIGEN3_VERSION_STRING etc

A number of CMake variables have been removed in commit f2984cd0 of Apr 27, 2021.

Allegedly, they are deprecrated. Not sure about this. I am frequent contributor to the CMake docs, yet I am not aware of any rule that deprecates such variables.

Anyway, users need some way to print the version of Eigen, and to refer to the include directory. For the latter, I found the solution

    get_target_property(EIGEN3_INCLUDE_DIR Eigen3::Eigen INTERFACE_INCLUDE_DIRECTORIES)

Yet I see no advantage in forcing users to write that extra line. For the version, I am clueless. The file Eigen3ConfigVersion.cmake, despite its name, does not seem to export any version information.