1. avoid decreasing all median nodes.
2. in __compute_initial_node_labels(), stop appending elements to median_labels when all node_labels are already selected.
3. return False directly in __update_node_label() when node_labels is empty.
4. solve the bug that median is not modified in place in __delete_node_from_median().

2. fix bugs for load_tud.

2. update load_tud function.
3. update MedianPreimageGenerator.

2. update README.md.
3. update preimage module, class Dataset.
4. update requirements.
5. add helper function to compute Gram matrix for each class.

2. add helper function generate_median_preimage_by_class in gklearn.preimage.utils.

2. add gedlib module.
3. add ged module.
4. create Kernel class and StructrualSP class.
5. rearrange directory.