Enhancement: `SceneCollection.get_values()` and equivalent for `is_bandstack()` in `SceneCollection` class

[lukasValentin]

Building on the insights gained from #71 two new features are proposed for the SceneCollection class:

1. SceneCollection.get_values(): This method will return the values in the SceneCollection as 4d tensor with shape (n_timestamps, n_bands, nrows, ncolls).
2. SceneCollection.is_bandstack(): This method will behave as RasterCollection.is_bandstack().