OptimizationSolver cleanup #277
Description
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