Investigate: swap pointer instead of copying fields

After using a solver like Advection Solver it is necessary to copy the solution into the temporary fields. The function call `advect(Field out, Field in, [...])` consists of the ourput field where the calculated solution is stored and the input field where the current solution is stored. to pass the solution to the next solver the current solution has to be as well in the temporary field, which will be the next in Field.