'Fetch `count` bytes of data offset by `start` bytes using `obj.id`.
If the object does not exist raises `ObjectNotFound`.
:type start: int
:param start: Set the position to start reading the dataset file
:type count: int
:param count: Read at most `count` bytes from the dataset'

def get_data(self, obj, start=0, count=(-1), base_dir=None, extra_dir=None, extra_dir_at_root=False, alt_name=None, obj_dir=False):

    raise NotImplementedError()
