OptimizationSolver cleanup

[phstratmann]

User story

As a developer, I want to have an OptimizationSolver that has no technical debt and thus allows for easy maintenance and dev work.

Conditions of satisfaction

• [ ] internally forward full cost matrices, and not cost_diagonal and cost_off_diagonal separately
• [ ] Provide interface to all variables at the level of the OptimizationSolver
• [ ] Identify and remove unnecessary hierarchies / SubProcModels
• [ ] Re-instantiate the CPU backend
• [ ] Clean up unit tests: remove unrequired ones, create performance unit tests