add parameter server and federated learning

eval/plot.py

eval/run.sh

eval/run_xtimes.sh

eval/run_xtimes_celeba.sh

eval/run_xtimes_cifar.sh

eval/run_xtimes_femnist.sh

eval/run_xtimes_reddit.sh

eval/run_xtimes_shakespeare.sh

eval/testingFederated.py

eval/testingPeerSampler.py

logs/config_celeba_sharing.ini

src/decentralizepy/node/DPSGDNodeFederated.py

src/decentralizepy/node/DPSGDNodeWithParameterServer.py

src/decentralizepy/node/FederatedParameterServer.py

src/decentralizepy/node/ParameterServer.py