Make stand-alone postprocessor

[Øyvind Evju]

The logic in the postprocessing framework does not depend on dolfin, and could be useful as a stand-alone, perhaps in its own repository.

This involves moving save and plot functionality out of nspostprocessor.py to e.g. a Save-class and Plot-class. It would also be useful to restructure PPField to keep a superclass independent of dolfin.

[Øyvind Evju]

Now in cbcpost