GitLab

- add pure virtual destructor to DataDescriptor
- create VolumeDescriptor which takes over most functionality from DataDescriptor
- move static function bestCommon(...) to a free function in DescriptorUtils.h/cpp
- fix other classes and tests to return and handle VolumeDescriptors instead of DataDescriptors

Add helper to print type in tests; Remove duplicate function of random generation of eigen vectors; Add test, which should fail sanitizer

For performance reasons this was removed, includes:
- removing functionality from DataHandlerCPU
- added simple benchmark
- adapting clang-tidy to not show warnings from catch2 testing

Build image hierarchy. Using install scripts in the scripts/ folder, buildDockerContainers.sh also runs tests on the containers.
Added an image for static analysis and documentation builds. Added hidden builds in .gitlab-ci.yml for local testing, which build and run tests.

Revert "Removed aliases of Eigen::VectorX, also in tests (as it will not work with older Eigen versions)."

This reverts commit 81572177

- add BlockLinearOperator to operators
- add block descriptors (IdenticalBlockDescriptor, PartitionDescriptor, RandomBlocksDescriptor) and made BlockDescriptor abstract

Timing should always be done using `std::steady_clock` because it's the
only clocked that guarantees steady advancement of `std::time_point`.