Feature/opt refactoring

Sorry for the big PR. The commits should be fairly self-contained and descriptive.

Added const wherever I found applicable
Replaced override with final wherever applicable
Replaced vector.at(i) with vector[i] and assertions
Changed from shared pointers to unique pointers (Invariants and IntViews are owned by the Engine and will now be returned by reference)
Added the macro definition CBLS_TEST in order to skip testing functionality when benchmarking/not testing
added [[nodiscard]] where applicable.
Accepted typing suggestions of CLion IDE.

astra-uu-se / atlantis