Fix calculation of L2 error in angeli test

See merge request !1411

(cherry picked from commit 0e52f00c)

c23a82f9 [test][freeflow] Call postTimeStep before timeLoop->advanceTimeStep
6fae9dc3 [test][freeflow] Let angeli problem have time_ and timeStepSize_ instead of timeLoop_.
3c90c74f [test][freeflow] Remove unused function sourceAtPos.
d694585f [test][freeflow] Remove function shouldWriteRestartFile.
f5959801 [test][freeflow] Move contents of function postTimeStep from problem to main for test angeli.
9bd243b9 [test][freeflow] Move vtk output vectors from problem to main file