Keep track of hierarchy depth when recursively scanning the user folder.

Started implementing issue #2.
Composite datasets are now recognized but not processed.

The code is in place to recognize and add composite datasets to the data tree, but currently all files in the composite datasets are aded to the list of invalid files since there are no dedicated processors yet.
Merge remote-tracking branch 'aarpon.ath.cx/feature/composite_datasets' into devel