Get rid of `Dataset.load_dataset_obj` in favor of `from_dataset_object`

Following up in the discussion from !142 (merged) ("Resolve "Dataset: JSON serialization""), we should consider changing Dataset to only load via the from_dataset_object method:

• @alessandro started a discussion: (+2 comments)

  Could we get rid of this method (i.e., load_dataset_obj), and only rely on the class method from_dataset_object? Ideally, this method would take the root_dir, and search for the dataset object file, based on self.filename_dataset, load it with json or pickle based on the extension.