Refactored Eigen / MKL / OpenMP setup

Introduced new CMake option OGS_EIGEN_PARALLEL_BACKEND which is OpenMP by default. When using OGS_USE_MKL=ON then the backend is MKL.

Removed options:

• OGS_USE_MKL_EIGEN_OPENMP
• OGS_EIGEN_USE_MKL

Please note when using OGS_USE_MKL=ON you can also set OGS_EIGEN_PARALLEL_BACKEND=OpenMP.

Also please note that when OGS_USE_MKL=ON and OGS_EIGEN_PARALLEL_BACKEND=MKL the OpenMP-parallelized assembly gets disabled!

Applies logic from !4156 (merged) to serial configs too. Also updated Eigen to a recent master commit (there is no new released version yet).

May fix #3462 (closed).

1. Feature description was added to the changelog
2. Any new feature or behaviour change was documented?