Select different training/dataset on clients

[eyp]

After #8, there's the possibility to choose between different training/datasets. Add the option in the client's page on the server dashboard to change it, so when the next training is run, those clients will do a different training.

Training option must be sent when a training is requested to a client.

[eyp]

Done. Now the central node dashboard allows to select different model trainings. For now there are 2 trainings: MNIST and Chest X-Ray.