Time predictions can be negative

[cpmpercussion]

The MDN model inside the IMPS system can potentially generate negative or zero time predictions (although these have probably been rare in my usage).

In particular the request_rnn_prediction function in predictive_music_model.py does nothing to prevent this.

https://github.com/cpmpercussion/imps/blob/5707ca01d4004d11603a9969276591c8ab5c28a4/predictive_music_model.py#L136

The best thing to do here is probably to adjust request_rnn_prediction to have a minimum bound on time prediction (probably zero) similarly to the approach in proc_generated_touch in the model