[test][freeflow] Move createAnalyticalSolution to file and reuse for several tests.

Merged Melanie Lipp requested to merge feature/freeflowCreateVTKAnalyticalSolutions into master

So far, every Navier-Stokes test with analytical solution implements its own function to prepare and get the analytical solution for the vtk output. This MR moves this functionality to one common class to reduce code duplication.

Edited by Melanie Lipp

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