Check if set values of block is persistent

Currently, selected block parameters, model_parameters and workspace_variables are reset with every _reset. I could imagine that these values are persistent because of the SimulationInput object only used as reference for the simulation and not as a place where the actual simulation values are stored. This would mean that these values only have to be set once and only if they have to change. This would make the resetting in _reset not necessary any more. This would help also for #97.