Refactoring: Quantities are now part of the pde_solver.

This has several benefits:
 - One can now conveniently output all quantities to VTK with just one call
 - Faster assembly (quantities are accessed with a simple index operation)
 - Reduced compilation times (approx. 2x), because use of ViennaData storage is reduced
 - No global state/storage. Everything gets cleaned up properly.
 - Known quantities (doping, etc.) and unknown quantities (potential, n, p, etc.) are handled using the same formalism.

Some cleanup of the code base still needs to be performed.