Calculate the total free energy and output it and its components
Lines 844-866.
Belongs in postTimeStep().
We may want to pull some of the code where these components are calculated into here. It might be slightly more efficient to calculate them up in takeTimeStep(), but strictly speaking we shouldn't need to calculate/output these every time step so pulling them out might make sense. Anything that seems too hard to pull out can stay where it is for now.
This is worth testing to make sure that the MPI reductions are done correctly and that the output file is written correctly.