Support integer / decision variables for OptimizationProblem / Optimizer

[schmoelder]

Currently, we only support continuous variables for optimization problems. However, there are many scenarios in which the variables to be optimized are integers (e.g. the number of axial cells in a Column).\

To do

• Support integers in OptimizationProblem (check how other libraries allow specification, e.g. Ax).
• Add flags for OptimizerBase and its implementations whether they support integer variables.