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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

Protect copy constructors in non-leaf and leaf classes, to avoid the implicitly generated ones by the compiler that could cause slicing. In Cloneable, the assignment operator was deleted to disable issues with implicitly generated assingment operators in derived classes to prevent slicing. (resolves #31)

add elsa:all convenience target and matching elsa.h convenience header for linking/including all of elsa. In addition: tests are no longer part of the default target, but have their own tests target. (#25)

Add DataHandlerMapCPU, move copy-on-write functionality to DataHandler, add block functionality to DataContainer