GroupTree initially empty on data change

[anders-kiaer]

Following #535, the multiple-tree-on-data-change issue is resolved, however initially (before the time slider is moved) there is no tree shown.

Expected behaviour:
The new tree is shown immediately on data change.

Related question:
On data change the time slider could either reset to default value (first value) or change to time step value closest to selected time step in the previous data set. What do you think is best @nilscb and @lindjoha? Reset to default value is perhaps easiest and enough at least at the current stage?