bugfix where I had indentation wrong and it wasn't checking for z values. Also, now I check that initial z location is at least a metre above the bottom. Doing sometimes the linear interpolation says that the drifter is above the bottom, but since we floor the fractional index later (in prepare_for_model_run), this might still end up being below the last w-vertical point
merging check_points branch after fixing checking of z0.