Remove primaryVariableName from model traits

As briefly discussed in !1188 (merged), the functions returning the name of the primary variables needed for reading-in vtu files (restart) should probably not be part of the model traits.

For the staggered discretization, one has to distinguish between primaryVariableNameFace() and primaryVariableNameCellCenter() while, e.g., the NavierStokes model traits per se should be independent of discretization.

A free standing function (maybe also in the form of property) might be more flexible. This should be discussed and implemented during the next Dumux day.