Google test for unit tests

What does this MR do?

Use Google Test as unit test system. According to multiple recommendations, even by Homebrew, I included the software as Git Submodule. The unit test for SimulationBase was updated to use Google Test.

This is a follow-up to !155 (closed) which has been marked Won't merge because we agreed in #152 (closed) not to introduce Boost as dependency to DORiE.

Further things to do

• CMake supports discovering unit tests after they have been compiled.

  The discovered tests are split into several granular targets, such that CTest reports the errors more granuarly. I don't know if this clashes with our naming convention for system and unit tests. Will investigate a bit.

  Result: In our testing pipeline, splitting each test executable into further CTest tests is of very limited use.

Is there something that needs to be double checked?

No.

Can this MR be accepted?

• Implemented unit test framework Google Test
• Adapted single test
• Pipeline passing
• Added Doxygen docs on unit tests
• Added entry to CHANGELOG.md

Related issues

Closes #152 (closed)