Prevent cyclic imports provided by user

[ferhaterata]

This enhancement is caused by #81

[ferhaterata]

Import relation should form a DAG (Directed Acyclic Graph) in Instances.
InstanceImport may form a cycle only once, but cannot form a cycle more than once. This exception occurs when a model imports an instance that conforms to itself. Thus, the instance also imports model in the ModelImport section. The reason why we need to import an instance into a metamodel is to provide an instance specification (partial instance) to specify an initial state or a possible state of the system. We have not added the required language constructs for that. However, in annotations user is able to reference an instance, that is may be the instance that conforms the model in which the annotation resides.