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.