Combine multicore and singlecore calculations?

[FrankvRaffe]

The current status of parallel calculations requires a call to rc_sim_multi while sequential calculations are done via rc_sim. This means that both functions need to be constantly updated in parallel with regards to inputs to prevent errors. Think whether a better solution is to create a function that gives the user the option to use singlecore or multicore and acts as router to the internal functions for it. This would prevent maintance on ensuring the functions behave exactly the same.