Add possibility to disable saving of data

[jgosmann]

Sometimes I don't want to save the data. For example in test runs to see wether the model runs successfully at all, but I don't care about the exact data. Also, I tend to run larger number of trials with psyrun on Sharcnet where I call the PyTry class from code and take care myself of all data saving.

[tcstewar]

You can get this behaviour right now with --data_dir None (or data_dir=None from Python). I'm not quite sure this is the best syntax (since it means that you can't set the data_dir to be "None" (the string) from the command line, which is a little odd, but I can't see anyone actually wanting to do that....