AskTellOptimizer needs an instances of TrainableProbabilisticModel

[pio-neil]

The AskTellOptimizer needs instances of TrainableProbabilisticModel even if no training is being performed by the optimizer.

In other words, if only the ask method is called, and not the tell method, then having a trainable model seems unnecessary, but this is required by the type declaration.

In our case we train models outside of Trieste, so it would be nicer if we could specify just a ProbabilisticModel if we only intend to use the ask method, and not the tell method.