Add growing model sharing

eval/config_celeba_grow.ini

eval/config_femnist_grow.ini

src/decentralizepy/datasets/Celeba.py

src/decentralizepy/datasets/Femnist.py

src/decentralizepy/sharing/GrowingAlpha.py

src/decentralizepy/sharing/PartialModel.py

src/decentralizepy/sharing/Sharing.py