`ALGEBRA_PLUGINS_INCLUDE_EIGEN` but `ALGEBRA_PLUGINS_SETUP_EIGEN3`

[wermos]

The option for including Eigen as a backend is called ALGEBRA_PLUGINS_INCLUDE_EIGEN but the option for setting up the library is called ALGEBRA_PLUGINS_SETUP_EIGEN3.

We should stick with a consistent name. Either Eigen3 everywhere or just Eigen everywhere.