Pass previous solution pointer to instationary assembler constructor
A pointer (preferably a shared_ptr) to the previous solution could be already passed to the constructor of the instationary assembler. In fact that should be obligatory so you can't forget. We could still have setPreviousSolution (preferably with a shared_ptr) to change it later. This would also get rid of the error handler that checks if a previous solution has been set if a timeLoop is present.