Validation

So I finally decided to make this a pull request before it grows even bigger. Sorry for the size, but at least its broken down into many tiny commits. :-)

This is close to the final version of the entity validation: I have little more that I could add without further input. In particular I would appreciate input on what to validate in / on: Block & Feature entities.

The following changes go beyond just the validation and might need discussion:
-As proposed in the issues I have implemented ```as*Dimension``` type specific getter in the ```Dimension``` entity. Therefore changing the order in which the classes get defined in ```Dimension.hpp``` was necessary.
-As proposed in the mailing list in some classes I split the multi-getter (e.g. ```references```) into two overloads instead of one method with default parameter to allow passing it to the validation.
There is lots of code duplication in `isValidUnit`, `isAtomic` and `isCompound`. Maybe a templated base struct?