Use smaller eigen imports

Many files so far include <Eigen/Dense> or <Eigen/Eigen>. But according to the Eigen website these headers include large parts of the Eigen library, which is seldom needed. Instead <Eigen/Core> is usually sufficient.

This MR replaces the includes of <Eigen/Dense> and <Eigen/Eigen> with <Eigen/Core> and adds the necessary additional Eigen headers where they are needed.

That might save a bit compile time, but I didn't measure that.

One disadvantage is that cross, determinant and inverse (among others) are not fully implemented in <Eigen/Core>, but need <Eigen/LU> or <Eigen/Geometry>, and sometimes only the linker will tell if the definition is missing:

ld.lld: error: lib/libMeshLib.so: undefined reference to Eigen::MatrixBase<Eigen::Matrix<double, 3, 1, 0, 3, 1> >::cross_product_return_type<Eigen::Matrix<double, 3, 1, 0, 3, 1> >::type Eigen::MatrixBase<Eigen::Matrix<double, 3, 1, 0, 3, 1> >::cross<Eigen::Matrix<double, 3, 1, 0, 3, 1> >(Eigen::MatrixBase<Eigen::Matrix<double, 3, 1, 0, 3, 1> > const&) const [--no-allow-shlib-undefined]
ld.lld: error: lib/libGeoLib.so: undefined reference to Eigen::MatrixBase<Eigen::Matrix<double, 3, 1, 0, 3, 1> >::cross_product_return_type<Eigen::Matrix<double, 3, 1, 0, 3, 1> >::type Eigen::MatrixBase<Eigen::Matrix<double, 3, 1, 0, 3, 1> >::cross<Eigen::Matrix<double, 3, 1, 0, 3, 1> >(Eigen::MatrixBase<Eigen::Matrix<double, 3, 1, 0, 3, 1> > const&) const [--no-allow-shlib-undefined]
ld.lld: error: lib/libMathLib.so: undefined reference to Eigen::MatrixBase<Eigen::Matrix<double, 3, 1, 0, 3, 1> >::cross_product_return_type<Eigen::Matrix<double, 3, 1, 0, 3, 1> >::type Eigen::MatrixBase<Eigen::Matrix<double, 3, 1, 0, 3, 1> >::cross<Eigen::Matrix<double, 3, 1, 0, 3, 1> >(Eigen::MatrixBase<Eigen::Matrix<double, 3, 1, 0, 3, 1> > const&) const [--no-allow-shlib-undefined]
collect2: error: ld returned 1 exit status