TrajectoryComputation: Only allow time steps and ensemble members where u, v and w are present

Closes #407 (closed).

In the current implementation, only the u wind variable is checked when collecting available init times, valid times and ensemble members for computing trajectories. The use case 2 data set has u, v and w only for the first 7 days, and after that w is not present anymore. That lead to a crash. This MR changes the computation of available trajectories by checking all three wind fields instead of just one.