Implement non-elementwise-constant permeability in convection diffusion

So far, the convection diffusion operators silently evaluate the permeability at the cell centers. This should be done at each quadrature point instead, if indicated by the parameter interface. I talked to @smuething and we propose the following interface for specifying it in the parameter interface:

static constexpr bool permeabilityIsCellwiseConstant(const Element& e)
{
  return false;
}

This would also allow people to drop the constexprness and vary the evaluation policy throughout the domain.