Add option to not compute predictions

[nicholasjclark]

Predictions in generated quantities can take up huge memory for large datasets. Need an argument in mvgam() to turn these off. Best way would be to remove from the model file, though this will force a re-compile if the same model is used without turning off. But nmix() will require a different approach. Will also require they be added back in for certain functions, such as hindcast()