std::vector<std::size_t> _ids_of_active_elements.

 improvement in SerialExecutor

due to its std::unique_ptr type instance.

also added two includes to ogs.cpp and ProjectData.cpp due to the forward declaration of the related classes.

[Jenkins] Use new xunit()-syntax.

Defined e.g. on win1: 16.

Use NUM_THREADS also for ctest parallelism.

https://github.com/ufz/jenkins-pipeline-library/commit/0f48380b1523fc626a72d3e85795aa94de29985e

Conversion happened in the constant vectorXi construction.

Allow C++17 language features.

This forcefully brings updates to Eigen-3.3, gtest as a submodule, and changed cmake scripts.

In multi-threaded environments multiple ctests running
parallel fight for the same cpu threads otherwise.

Failing on mac now with error 9.2e-7 since c++17/Eigen-3.3 change.

MacOS repeatedly could not finish in 3h.

The original initialization of the std::array's was by copy anyway
and somehow caused in seg faults on mac after adding c++17 standard.