GP creation should be independent of prediction type

[odunbar]

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.