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.

This reverts commit a8948a35b2.

This reverts commit 7ab225d330.

This reverts commit f1d4a6e5e3.

This reverts commit 5fe81a932b.