[freeflow][l2error] Get analytical solution from the corresponding function
What this MR does / why does DuMux need it:, not from the Dirichlet conditions. I would as a user not expect that I have to define dirichletAtPos to return analyticalSolution. I might in some cases consider myself fine with dirichletAtPos as zero velocities if they are zero at the boundary and nonzero within the domain. Then L2error would return wrong results.