[pm][ff] Improve indices, move into model traits, [pm] free volume variables from type tag

• Moves indices into modeltraits

• Removes TypeTag dependency from privarswitches

• Removes TypeTag dependency from pm volvars

• Cleans up vtk output fields

• Introduces variable wettability for 2-phase models

• Generalize fluid system indices not to fix wetting and non-wetting phase

• NavierStokesIndices

• export Indices type and numPhases() & numComponents() in vol var base class

• use numPhases() and numComponents() from vol vars in vtk output fields so that the loops are correct

• -> this should leave output fields without template parameters.. verify!

• implement static_asserts in volume variables to make sure that the specifications of the model traits are in agreement with whatever model the vol vars are implementing

See #448 (closed).