Move computation of adaptive dt before read

[BenjaminRodenberg]

Currently, the computation of the adaptive solverDt happens after the read. I would argue that it should happen before the read. I guess the original idea here was that the computation of the solverDt depends on the quantity that was read. However, I think this does not make sense for the following two reasons:

1. If we use waveforms, we need the dt before reading, not after. See also v3: Make dt mandatory in documentation #257
2. If the adaptive solverDt depends on the quantity read from preCICE, then we need to read that quantity also at the right location in time. This leads somehow to a deadlock, that we could, for example, solve by iterating:

bool solverDtChanging = true;
solverDt = beginTimeStep(displacements); // e.g. compute adaptive dt that depends on displacements
while(solverDtChanging) {
  dt = min(preciceDt, solverDt);
  precice.readBlockVectorData(displID, vertexSize, vertexIDs, dt, displacements);
  oldSolverDt = solverDt;
  solverDt = beginTimeStep(displacements);
  solverDtChanging = check(solverDt, oldSolverDt, tol);
}

[uekerman]

Makes sense