[WIP] Collect python plotting functionality in dune-grid.

At the moment, we have two plotting modules for the python bindings in DUNE. The first is in dune.common.plotting, the second in dune.fem.plotting, and both have pretty similar code.

We should merge those module and move the code to the right place, which is actually dune.grid.plotting.

I discussed with @andreas.dedner and @robert.kloefkorn that we should also cleanup the module and simplify the routines and interfaces. The methods where implemented as convenience methods, grew over time and many people have added functionality depending on their personal needs.

To get started, I collected what is implemented so far and now propose simplified function signatures in a new plotting module.

Let's open the discussion, if this is what we want to expose from the DUNE python side.