Add bool in model indicating whether it is trained

[benjamc]

is_not_trained = isinstance(self._model, GaussianProcess) and not self._model._is_trained \ or isinstance(self._model, RandomForest) and self._rf is None

[benjamc]

add model_trained as property to surrogate model abstract class (or is_model_trained() function)

[timruhkopf]

PR #1232 closes this