GridSearchCV and other metaoptimzer should allow an other Optimizer as input

[AKuederle]

At the moment GridSearchCV allows to accepts a pipeline as input that it then wraps internally in Optimize. As a result, we need to forward parameters to Optimize and there is no way for a user to wrap another Optimizer into GridSearchCV.

A solution could be to allow passing a Optimizer instance to GridSearchCV directly. The only downside is, that you would need to nest the naming for the Parameter Grid one level deeper, which can become annoying

[AKuederle]

Maybe we could add a "prefix_parameter_names" attribute, that could be set, if you pass a complicated chain of nested Optimizers. Might also be helpful in case of #84

[AKuederle]

I decided against this for now, as this might make things much more complicated. However in #116 I extracted the part of the GridSearchCV method, where we wrap the pipeline into a separate function. So in case someone really needs it, they could subclass GridSearchCV to get another type of optimizer working