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.