Cox model support not working

[deaneckles]

It seems like model='cox' is not actually supported because predict.sgd doesn't know how to handle it, despite sgd accepting it.

https://github.com/airoldilab/sgd/blob/a256063e7add0ec43f051c2b0ba5a959e950dceb/R/predict.sgd.R#L23-L26

This then means that the example in https://github.com/airoldilab/sgd/blob/master/demo/cox-regression.R does not work.

Looking forward to trying this nice package with some continuous time data. Thanks!