Postprocessor

[Rombur]

Clean up post-processor to output solution.

[dalg24]

@Rombur Would you comment that issue? #242 added an inspector that can trigger the post processor to write the solution to a file. What is the next action?

[Rombur]

The post-processor is not tested and I have no clue if it actually works. The inspector just output the mesh because it's what I needed. It also calls the post-processor but that is legacy code that you didn't want to lose and it has never been tested.

[Rombur]

Just to be clear on what the clean up is about. We decided to break the Postprocessor class in multiple classes. The problem with the current class is that Postprocessor needs to compute some quantities before we can start a simulation, i.e., it is not only a postprocessor.