Check for Workflow Circles

[kantale]

So far we allow for a workflow A to include a workflow B that includes the Workflow A. This creates a circle in the Workflow belongsto Workflow graph which has the following effects:

• Workflows cannot be deleted (a workflow cannot be deleted if it is a part in another workflows)
• Workflows cannot be imported (we need to have a clear workflow order in order to be able to import workflows)

Solution is: check if a workflow already exists in a graph before importing it.