Easier native access to Record

While working on the Record based definitions, I noticed that there is a clear redundancy in the get*() methods: they pop up in Record, Config, and now I also needed them in a utility class that manipulates model definition data stored in a Record.

The solution was to add a new class called RecordAccessor that contains nothing but these get*() methods. This new class can then be inherited into any other class that benefits from providing native access to a Record.

While RecordAccessor is currently specific to Record instances, it could still be generalized into an IAccessor class that queries values from an abstract source where values are addressed using paths — this would make it possible to use it also with PathTree.