Conditional GAN support in `tune()`

[tsrobinson]

We had a previous issue (#7) on updating tune() that was largely resolved by #27.

One outstanding change is to add conditional GAN support to the hyperparameter search pipeline.

Since critic_loss simply returns the final loss value, we don't need to worry about changing anything to do with test_func. Instead, we just need to accommodate the conditional labels being parsed to sygnet_model within the tune procedure itself.