[discretization] specialize evalSolution for primary variables with state

Interpolating the entries of primary variable vectors by means of shape functions can be wrong: If the vectors have different states, the entries can be associated with different physical fields. Since the function evalSolution is physics-agnostic, this presents a simple solution without adding knowledge about the physics. If the states are different over all corners of the associated geometry, evalSolution returns the primary variables associated with the vertex that is closest to the evaluation point.