data paths - fix bugs & revise how the root is specified

1. Error to fix: occurs when trying to pass a specific datapath at Dataset object initialization:

Runtime error (Exception): Traceback (most recent call last):
...\src\aaad\data_objects\data_structures.py", line 530, in __init__
    self.filename_dataset = os.path.join(self.datapath, name.replace(" ", "_"))
...\src\aaad\data_objects\data_structures.py", line 706, in datapath
    return self._datapath
AttributeError: 'Dataset' object has no attribute '_datapath'

2. Revise the defaults of datapath and logpath, esp. how the root is specified (dependency on HOME path) -> following up on #20 (closed)