GP creation should be independent of prediction type

We currently create the GP with a `prediction_type` in the constructor of the `GPObj` constructor in `GPEmulator.jl`
Though a prediction type should be specified by the user - it should be done at runtime of the `GPObj.predict` function. We want it defined at run time because the GP training is independent of the prediction type, and the same trained GP can be used for both types of prediction `Ytype` and `Ftype`. Furthermore the type of prediction shouldn't be stored for these same reasons.