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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`.

  * using underlying Eigen expression templates
  * scalar operations with expression templates
  * save DataContainer meta info in expressions
  * add unary operators
  * adding enum for DataHandlerMapCPU type
  * added expression templates readme
  * removed operators between DataContainers, scalars and DataHandlers
  * in-place operations using expressions
  * test cases
  * benchmark script