Callback for changing task generator mode between 'train', 'test' and 'val'

[tkarab]

• [x] s,g,r domains should be kept in a dictionary depending on the database and the experiment being done, and have the TaskGenerator class load them upon initialization
• [x] Setter function in the TaskGenerator class for changing experiment
• [x] setter function for changing mode ('train', 'test', 'val')
• [x] check if task generator mode, data intake etc can be set by a callback
• [x] callback function for changing mode, data_intake, experiment