Enforce parameters and feature definitions belonging together in feature definition graph

[FRosner]

The feature definition graph should check whether an inserted dependency is valid. A dependency is valid iff

• the parent feature is part of the graph already
• the child parameter belongs to the child feature distribution

The FeatureDefinition already has a method that returns all dependent parameters. An easy way to check the second condition would be to check whether the dependent parameter to be inserted is part of the dependent parameters of the feature definition.