Add Class based validation rules

[ewuerger]

On some classes we encounter special rules regarding some attributes which then need to be dealt with in for example the used Accessor's methods like __get__ and __set__, __delete__, ... (for the change API). This probably is a big reason why there are so many different Accessor's, looking at you CustomAccessor. The idea will be presented in a response by @Wuestengecko. Here follows a list of encountered special cases for notation:

• cs.Parts xmltag are non-unique and depend on their parent's class: If the parent is a ComponentPkg the xmltag is ownedParts, under Components it has ownedFeatures. The latter which is shared by ComponentPorts,
• exchanges have non-None .source and .target attributes else they are broken,
• A Component has atleast one Part,
• Some is_actor, is_abstract and is_human rules,
• ...

The implementation of validation rules could reduce the amount of Accessor's and lessen the complexity of our infrastructure.