Allow arrays, instance literals and stuff in initialisation of attribute definitions. Therefore, a large part of the instance language was merged into the base language.#25 #28

stuff that is optimiser algorithm-independent. We still have to move some more content back to
core.main while we implement new optimisers. #19 #15

Fixed regressions with domain-specific language parsing #18. This bug was introduced by d74325cb.

- Removed Name, NamedAttributeDefinition and UnnamedAttributeDefinition from dl-language and added a single AttributeDefinition (which references a definition)
- Renamed name attribute of instance and attribute to definition since it references the definition.
- Fixed bug in attribute lookup