Postprocessors should have densities and then some postprocessors can go away

[mangerij]

Some of the postprocessors should have corresponding aux density fields. Then we can get rid of having a specific postprocessor for every piece of physics.

Will simplify and compactify the code...

[mangerij]

...maybe even speed up... assuming aux field calculation and then postprocessor integration is quicker than a postprocessor integration from a predefined field. This was from an older question/note. Maybe can test this idea...